diff --git a/src/lib/es2015.core.d.ts b/src/lib/es2015.core.d.ts
index 8026b39455a17..15c637a873660 100644
--- a/src/lib/es2015.core.d.ts
+++ b/src/lib/es2015.core.d.ts
@@ -401,6 +401,7 @@ interface String {
      * same as the corresponding elements of this object (converted to a String) starting at
      * endPosition – length(this). Otherwise returns false.
      */
+    endsWith<T extends string>(searchString: T, endPosition?: undefined): this is `${string}${T}`;
     endsWith(searchString: string, endPosition?: number): boolean;
 
     /**
@@ -431,6 +432,7 @@ interface String {
      * same as the corresponding elements of this object (converted to a String) starting at
      * position. Otherwise returns false.
      */
+    startsWith<T extends string>(searchString: T, position?: 0): this is `${T}${string}`;
     startsWith(searchString: string, position?: number): boolean;
 
     /**
diff --git a/tests/baselines/reference/stringEndsWith.errors.txt b/tests/baselines/reference/stringEndsWith.errors.txt
new file mode 100644
index 0000000000000..a4b2031872b74
--- /dev/null
+++ b/tests/baselines/reference/stringEndsWith.errors.txt
@@ -0,0 +1,28 @@
+tests/cases/compiler/stringEndsWith.ts(9,46): error TS2345: Argument of type 'string' is not assignable to parameter of type '`${string}test`'.
+tests/cases/compiler/stringEndsWith.ts(10,46): error TS2345: Argument of type 'string' is not assignable to parameter of type '`${string}test`'.
+tests/cases/compiler/stringEndsWith.ts(11,56): error TS2345: Argument of type 'string' is not assignable to parameter of type '`${string}test`'.
+tests/cases/compiler/stringEndsWith.ts(12,56): error TS2345: Argument of type 'string' is not assignable to parameter of type '`${string}test`'.
+
+
+==== tests/cases/compiler/stringEndsWith.ts (4 errors) ====
+    declare var test: string;
+    declare function requirePostfix(input: `${string}test`): void;
+    
+    // works
+    if (test.endsWith('test')) requirePostfix(test);
+    if (test.endsWith('test', undefined)) requirePostfix(test);
+    
+    // errors
+    if (test.endsWith('test', 0)) requirePostfix(test);
+                                                 ~~~~
+!!! error TS2345: Argument of type 'string' is not assignable to parameter of type '`${string}test`'.
+    if (test.endsWith('test', 1)) requirePostfix(test);
+                                                 ~~~~
+!!! error TS2345: Argument of type 'string' is not assignable to parameter of type '`${string}test`'.
+    if (test.endsWith('test', 0 as number)) requirePostfix(test);
+                                                           ~~~~
+!!! error TS2345: Argument of type 'string' is not assignable to parameter of type '`${string}test`'.
+    if (test.endsWith('test', 1 as number)) requirePostfix(test);
+                                                           ~~~~
+!!! error TS2345: Argument of type 'string' is not assignable to parameter of type '`${string}test`'.
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/stringEndsWith.js b/tests/baselines/reference/stringEndsWith.js
new file mode 100644
index 0000000000000..8510bca71c8ee
--- /dev/null
+++ b/tests/baselines/reference/stringEndsWith.js
@@ -0,0 +1,30 @@
+//// [stringEndsWith.ts]
+declare var test: string;
+declare function requirePostfix(input: `${string}test`): void;
+
+// works
+if (test.endsWith('test')) requirePostfix(test);
+if (test.endsWith('test', undefined)) requirePostfix(test);
+
+// errors
+if (test.endsWith('test', 0)) requirePostfix(test);
+if (test.endsWith('test', 1)) requirePostfix(test);
+if (test.endsWith('test', 0 as number)) requirePostfix(test);
+if (test.endsWith('test', 1 as number)) requirePostfix(test);
+
+
+//// [stringEndsWith.js]
+// works
+if (test.endsWith('test'))
+    requirePostfix(test);
+if (test.endsWith('test', undefined))
+    requirePostfix(test);
+// errors
+if (test.endsWith('test', 0))
+    requirePostfix(test);
+if (test.endsWith('test', 1))
+    requirePostfix(test);
+if (test.endsWith('test', 0))
+    requirePostfix(test);
+if (test.endsWith('test', 1))
+    requirePostfix(test);
diff --git a/tests/baselines/reference/stringEndsWith.symbols b/tests/baselines/reference/stringEndsWith.symbols
new file mode 100644
index 0000000000000..278a6a835a8b1
--- /dev/null
+++ b/tests/baselines/reference/stringEndsWith.symbols
@@ -0,0 +1,53 @@
+=== tests/cases/compiler/stringEndsWith.ts ===
+declare var test: string;
+>test : Symbol(test, Decl(stringEndsWith.ts, 0, 11))
+
+declare function requirePostfix(input: `${string}test`): void;
+>requirePostfix : Symbol(requirePostfix, Decl(stringEndsWith.ts, 0, 25))
+>input : Symbol(input, Decl(stringEndsWith.ts, 1, 32))
+
+// works
+if (test.endsWith('test')) requirePostfix(test);
+>test.endsWith : Symbol(String.endsWith, Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --))
+>test : Symbol(test, Decl(stringEndsWith.ts, 0, 11))
+>endsWith : Symbol(String.endsWith, Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --))
+>requirePostfix : Symbol(requirePostfix, Decl(stringEndsWith.ts, 0, 25))
+>test : Symbol(test, Decl(stringEndsWith.ts, 0, 11))
+
+if (test.endsWith('test', undefined)) requirePostfix(test);
+>test.endsWith : Symbol(String.endsWith, Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --))
+>test : Symbol(test, Decl(stringEndsWith.ts, 0, 11))
+>endsWith : Symbol(String.endsWith, Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --))
+>undefined : Symbol(undefined)
+>requirePostfix : Symbol(requirePostfix, Decl(stringEndsWith.ts, 0, 25))
+>test : Symbol(test, Decl(stringEndsWith.ts, 0, 11))
+
+// errors
+if (test.endsWith('test', 0)) requirePostfix(test);
+>test.endsWith : Symbol(String.endsWith, Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --))
+>test : Symbol(test, Decl(stringEndsWith.ts, 0, 11))
+>endsWith : Symbol(String.endsWith, Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --))
+>requirePostfix : Symbol(requirePostfix, Decl(stringEndsWith.ts, 0, 25))
+>test : Symbol(test, Decl(stringEndsWith.ts, 0, 11))
+
+if (test.endsWith('test', 1)) requirePostfix(test);
+>test.endsWith : Symbol(String.endsWith, Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --))
+>test : Symbol(test, Decl(stringEndsWith.ts, 0, 11))
+>endsWith : Symbol(String.endsWith, Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --))
+>requirePostfix : Symbol(requirePostfix, Decl(stringEndsWith.ts, 0, 25))
+>test : Symbol(test, Decl(stringEndsWith.ts, 0, 11))
+
+if (test.endsWith('test', 0 as number)) requirePostfix(test);
+>test.endsWith : Symbol(String.endsWith, Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --))
+>test : Symbol(test, Decl(stringEndsWith.ts, 0, 11))
+>endsWith : Symbol(String.endsWith, Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --))
+>requirePostfix : Symbol(requirePostfix, Decl(stringEndsWith.ts, 0, 25))
+>test : Symbol(test, Decl(stringEndsWith.ts, 0, 11))
+
+if (test.endsWith('test', 1 as number)) requirePostfix(test);
+>test.endsWith : Symbol(String.endsWith, Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --))
+>test : Symbol(test, Decl(stringEndsWith.ts, 0, 11))
+>endsWith : Symbol(String.endsWith, Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --))
+>requirePostfix : Symbol(requirePostfix, Decl(stringEndsWith.ts, 0, 25))
+>test : Symbol(test, Decl(stringEndsWith.ts, 0, 11))
+
diff --git a/tests/baselines/reference/stringEndsWith.types b/tests/baselines/reference/stringEndsWith.types
new file mode 100644
index 0000000000000..da03fe9e6fb29
--- /dev/null
+++ b/tests/baselines/reference/stringEndsWith.types
@@ -0,0 +1,77 @@
+=== tests/cases/compiler/stringEndsWith.ts ===
+declare var test: string;
+>test : string
+
+declare function requirePostfix(input: `${string}test`): void;
+>requirePostfix : (input: `${string}test`) => void
+>input : `${string}test`
+
+// works
+if (test.endsWith('test')) requirePostfix(test);
+>test.endsWith('test') : boolean
+>test.endsWith : { <T extends string>(searchString: T, endPosition?: undefined): this is `${string}${T}`; (searchString: string, endPosition?: number): boolean; }
+>test : string
+>endsWith : { <T extends string>(searchString: T, endPosition?: undefined): this is `${string}${T}`; (searchString: string, endPosition?: number): boolean; }
+>'test' : "test"
+>requirePostfix(test) : void
+>requirePostfix : (input: `${string}test`) => void
+>test : `${string}test`
+
+if (test.endsWith('test', undefined)) requirePostfix(test);
+>test.endsWith('test', undefined) : boolean
+>test.endsWith : { <T extends string>(searchString: T, endPosition?: undefined): this is `${string}${T}`; (searchString: string, endPosition?: number): boolean; }
+>test : string
+>endsWith : { <T extends string>(searchString: T, endPosition?: undefined): this is `${string}${T}`; (searchString: string, endPosition?: number): boolean; }
+>'test' : "test"
+>undefined : undefined
+>requirePostfix(test) : void
+>requirePostfix : (input: `${string}test`) => void
+>test : `${string}test`
+
+// errors
+if (test.endsWith('test', 0)) requirePostfix(test);
+>test.endsWith('test', 0) : boolean
+>test.endsWith : { <T extends string>(searchString: T, endPosition?: undefined): this is `${string}${T}`; (searchString: string, endPosition?: number): boolean; }
+>test : string
+>endsWith : { <T extends string>(searchString: T, endPosition?: undefined): this is `${string}${T}`; (searchString: string, endPosition?: number): boolean; }
+>'test' : "test"
+>0 : 0
+>requirePostfix(test) : void
+>requirePostfix : (input: `${string}test`) => void
+>test : string
+
+if (test.endsWith('test', 1)) requirePostfix(test);
+>test.endsWith('test', 1) : boolean
+>test.endsWith : { <T extends string>(searchString: T, endPosition?: undefined): this is `${string}${T}`; (searchString: string, endPosition?: number): boolean; }
+>test : string
+>endsWith : { <T extends string>(searchString: T, endPosition?: undefined): this is `${string}${T}`; (searchString: string, endPosition?: number): boolean; }
+>'test' : "test"
+>1 : 1
+>requirePostfix(test) : void
+>requirePostfix : (input: `${string}test`) => void
+>test : string
+
+if (test.endsWith('test', 0 as number)) requirePostfix(test);
+>test.endsWith('test', 0 as number) : boolean
+>test.endsWith : { <T extends string>(searchString: T, endPosition?: undefined): this is `${string}${T}`; (searchString: string, endPosition?: number): boolean; }
+>test : string
+>endsWith : { <T extends string>(searchString: T, endPosition?: undefined): this is `${string}${T}`; (searchString: string, endPosition?: number): boolean; }
+>'test' : "test"
+>0 as number : number
+>0 : 0
+>requirePostfix(test) : void
+>requirePostfix : (input: `${string}test`) => void
+>test : string
+
+if (test.endsWith('test', 1 as number)) requirePostfix(test);
+>test.endsWith('test', 1 as number) : boolean
+>test.endsWith : { <T extends string>(searchString: T, endPosition?: undefined): this is `${string}${T}`; (searchString: string, endPosition?: number): boolean; }
+>test : string
+>endsWith : { <T extends string>(searchString: T, endPosition?: undefined): this is `${string}${T}`; (searchString: string, endPosition?: number): boolean; }
+>'test' : "test"
+>1 as number : number
+>1 : 1
+>requirePostfix(test) : void
+>requirePostfix : (input: `${string}test`) => void
+>test : string
+
diff --git a/tests/baselines/reference/stringStartsWith.errors.txt b/tests/baselines/reference/stringStartsWith.errors.txt
new file mode 100644
index 0000000000000..3540ab1a4b72d
--- /dev/null
+++ b/tests/baselines/reference/stringStartsWith.errors.txt
@@ -0,0 +1,25 @@
+tests/cases/compiler/stringStartsWith.ts(10,47): error TS2345: Argument of type 'string' is not assignable to parameter of type '`test${string}`'.
+tests/cases/compiler/stringStartsWith.ts(11,57): error TS2345: Argument of type 'string' is not assignable to parameter of type '`test${string}`'.
+tests/cases/compiler/stringStartsWith.ts(12,57): error TS2345: Argument of type 'string' is not assignable to parameter of type '`test${string}`'.
+
+
+==== tests/cases/compiler/stringStartsWith.ts (3 errors) ====
+    declare var test: string;
+    declare function requirePrefix(input: `test${string}`): void;
+    
+    // works
+    if (test.startsWith('test')) requirePrefix(test);
+    if (test.startsWith('test', undefined)) requirePrefix(test);
+    if (test.startsWith('test', 0)) requirePrefix(test);
+    
+    // errors
+    if (test.startsWith('test', 1)) requirePrefix(test);
+                                                  ~~~~
+!!! error TS2345: Argument of type 'string' is not assignable to parameter of type '`test${string}`'.
+    if (test.startsWith('test', 0 as number)) requirePrefix(test);
+                                                            ~~~~
+!!! error TS2345: Argument of type 'string' is not assignable to parameter of type '`test${string}`'.
+    if (test.startsWith('test', 1 as number)) requirePrefix(test);
+                                                            ~~~~
+!!! error TS2345: Argument of type 'string' is not assignable to parameter of type '`test${string}`'.
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/stringStartsWith.js b/tests/baselines/reference/stringStartsWith.js
new file mode 100644
index 0000000000000..1cc8ede021bf4
--- /dev/null
+++ b/tests/baselines/reference/stringStartsWith.js
@@ -0,0 +1,30 @@
+//// [stringStartsWith.ts]
+declare var test: string;
+declare function requirePrefix(input: `test${string}`): void;
+
+// works
+if (test.startsWith('test')) requirePrefix(test);
+if (test.startsWith('test', undefined)) requirePrefix(test);
+if (test.startsWith('test', 0)) requirePrefix(test);
+
+// errors
+if (test.startsWith('test', 1)) requirePrefix(test);
+if (test.startsWith('test', 0 as number)) requirePrefix(test);
+if (test.startsWith('test', 1 as number)) requirePrefix(test);
+
+
+//// [stringStartsWith.js]
+// works
+if (test.startsWith('test'))
+    requirePrefix(test);
+if (test.startsWith('test', undefined))
+    requirePrefix(test);
+if (test.startsWith('test', 0))
+    requirePrefix(test);
+// errors
+if (test.startsWith('test', 1))
+    requirePrefix(test);
+if (test.startsWith('test', 0))
+    requirePrefix(test);
+if (test.startsWith('test', 1))
+    requirePrefix(test);
diff --git a/tests/baselines/reference/stringStartsWith.symbols b/tests/baselines/reference/stringStartsWith.symbols
new file mode 100644
index 0000000000000..b2e842ad202c4
--- /dev/null
+++ b/tests/baselines/reference/stringStartsWith.symbols
@@ -0,0 +1,53 @@
+=== tests/cases/compiler/stringStartsWith.ts ===
+declare var test: string;
+>test : Symbol(test, Decl(stringStartsWith.ts, 0, 11))
+
+declare function requirePrefix(input: `test${string}`): void;
+>requirePrefix : Symbol(requirePrefix, Decl(stringStartsWith.ts, 0, 25))
+>input : Symbol(input, Decl(stringStartsWith.ts, 1, 31))
+
+// works
+if (test.startsWith('test')) requirePrefix(test);
+>test.startsWith : Symbol(String.startsWith, Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --))
+>test : Symbol(test, Decl(stringStartsWith.ts, 0, 11))
+>startsWith : Symbol(String.startsWith, Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --))
+>requirePrefix : Symbol(requirePrefix, Decl(stringStartsWith.ts, 0, 25))
+>test : Symbol(test, Decl(stringStartsWith.ts, 0, 11))
+
+if (test.startsWith('test', undefined)) requirePrefix(test);
+>test.startsWith : Symbol(String.startsWith, Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --))
+>test : Symbol(test, Decl(stringStartsWith.ts, 0, 11))
+>startsWith : Symbol(String.startsWith, Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --))
+>undefined : Symbol(undefined)
+>requirePrefix : Symbol(requirePrefix, Decl(stringStartsWith.ts, 0, 25))
+>test : Symbol(test, Decl(stringStartsWith.ts, 0, 11))
+
+if (test.startsWith('test', 0)) requirePrefix(test);
+>test.startsWith : Symbol(String.startsWith, Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --))
+>test : Symbol(test, Decl(stringStartsWith.ts, 0, 11))
+>startsWith : Symbol(String.startsWith, Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --))
+>requirePrefix : Symbol(requirePrefix, Decl(stringStartsWith.ts, 0, 25))
+>test : Symbol(test, Decl(stringStartsWith.ts, 0, 11))
+
+// errors
+if (test.startsWith('test', 1)) requirePrefix(test);
+>test.startsWith : Symbol(String.startsWith, Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --))
+>test : Symbol(test, Decl(stringStartsWith.ts, 0, 11))
+>startsWith : Symbol(String.startsWith, Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --))
+>requirePrefix : Symbol(requirePrefix, Decl(stringStartsWith.ts, 0, 25))
+>test : Symbol(test, Decl(stringStartsWith.ts, 0, 11))
+
+if (test.startsWith('test', 0 as number)) requirePrefix(test);
+>test.startsWith : Symbol(String.startsWith, Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --))
+>test : Symbol(test, Decl(stringStartsWith.ts, 0, 11))
+>startsWith : Symbol(String.startsWith, Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --))
+>requirePrefix : Symbol(requirePrefix, Decl(stringStartsWith.ts, 0, 25))
+>test : Symbol(test, Decl(stringStartsWith.ts, 0, 11))
+
+if (test.startsWith('test', 1 as number)) requirePrefix(test);
+>test.startsWith : Symbol(String.startsWith, Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --))
+>test : Symbol(test, Decl(stringStartsWith.ts, 0, 11))
+>startsWith : Symbol(String.startsWith, Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --))
+>requirePrefix : Symbol(requirePrefix, Decl(stringStartsWith.ts, 0, 25))
+>test : Symbol(test, Decl(stringStartsWith.ts, 0, 11))
+
diff --git a/tests/baselines/reference/stringStartsWith.types b/tests/baselines/reference/stringStartsWith.types
new file mode 100644
index 0000000000000..688e18b3c61d3
--- /dev/null
+++ b/tests/baselines/reference/stringStartsWith.types
@@ -0,0 +1,77 @@
+=== tests/cases/compiler/stringStartsWith.ts ===
+declare var test: string;
+>test : string
+
+declare function requirePrefix(input: `test${string}`): void;
+>requirePrefix : (input: `test${string}`) => void
+>input : `test${string}`
+
+// works
+if (test.startsWith('test')) requirePrefix(test);
+>test.startsWith('test') : boolean
+>test.startsWith : { <T extends string>(searchString: T, position?: 0): this is `${T}${string}`; (searchString: string, position?: number): boolean; }
+>test : string
+>startsWith : { <T extends string>(searchString: T, position?: 0): this is `${T}${string}`; (searchString: string, position?: number): boolean; }
+>'test' : "test"
+>requirePrefix(test) : void
+>requirePrefix : (input: `test${string}`) => void
+>test : `test${string}`
+
+if (test.startsWith('test', undefined)) requirePrefix(test);
+>test.startsWith('test', undefined) : boolean
+>test.startsWith : { <T extends string>(searchString: T, position?: 0): this is `${T}${string}`; (searchString: string, position?: number): boolean; }
+>test : string
+>startsWith : { <T extends string>(searchString: T, position?: 0): this is `${T}${string}`; (searchString: string, position?: number): boolean; }
+>'test' : "test"
+>undefined : undefined
+>requirePrefix(test) : void
+>requirePrefix : (input: `test${string}`) => void
+>test : `test${string}`
+
+if (test.startsWith('test', 0)) requirePrefix(test);
+>test.startsWith('test', 0) : boolean
+>test.startsWith : { <T extends string>(searchString: T, position?: 0): this is `${T}${string}`; (searchString: string, position?: number): boolean; }
+>test : string
+>startsWith : { <T extends string>(searchString: T, position?: 0): this is `${T}${string}`; (searchString: string, position?: number): boolean; }
+>'test' : "test"
+>0 : 0
+>requirePrefix(test) : void
+>requirePrefix : (input: `test${string}`) => void
+>test : `test${string}`
+
+// errors
+if (test.startsWith('test', 1)) requirePrefix(test);
+>test.startsWith('test', 1) : boolean
+>test.startsWith : { <T extends string>(searchString: T, position?: 0): this is `${T}${string}`; (searchString: string, position?: number): boolean; }
+>test : string
+>startsWith : { <T extends string>(searchString: T, position?: 0): this is `${T}${string}`; (searchString: string, position?: number): boolean; }
+>'test' : "test"
+>1 : 1
+>requirePrefix(test) : void
+>requirePrefix : (input: `test${string}`) => void
+>test : string
+
+if (test.startsWith('test', 0 as number)) requirePrefix(test);
+>test.startsWith('test', 0 as number) : boolean
+>test.startsWith : { <T extends string>(searchString: T, position?: 0): this is `${T}${string}`; (searchString: string, position?: number): boolean; }
+>test : string
+>startsWith : { <T extends string>(searchString: T, position?: 0): this is `${T}${string}`; (searchString: string, position?: number): boolean; }
+>'test' : "test"
+>0 as number : number
+>0 : 0
+>requirePrefix(test) : void
+>requirePrefix : (input: `test${string}`) => void
+>test : string
+
+if (test.startsWith('test', 1 as number)) requirePrefix(test);
+>test.startsWith('test', 1 as number) : boolean
+>test.startsWith : { <T extends string>(searchString: T, position?: 0): this is `${T}${string}`; (searchString: string, position?: number): boolean; }
+>test : string
+>startsWith : { <T extends string>(searchString: T, position?: 0): this is `${T}${string}`; (searchString: string, position?: number): boolean; }
+>'test' : "test"
+>1 as number : number
+>1 : 1
+>requirePrefix(test) : void
+>requirePrefix : (input: `test${string}`) => void
+>test : string
+
diff --git a/tests/baselines/reference/taggedTemplateStringsWithOverloadResolution3_ES6.errors.txt b/tests/baselines/reference/taggedTemplateStringsWithOverloadResolution3_ES6.errors.txt
index 13147855b73b1..8a1ee136f1b68 100644
--- a/tests/baselines/reference/taggedTemplateStringsWithOverloadResolution3_ES6.errors.txt
+++ b/tests/baselines/reference/taggedTemplateStringsWithOverloadResolution3_ES6.errors.txt
@@ -113,7 +113,7 @@ tests/cases/conformance/es6/templates/taggedTemplateStringsWithOverloadResolutio
     fn5 `${ (n) => n.toFixed() }`; // will error; 'n' should have type 'string'.
                      ~~~~~~~
 !!! error TS2551: Property 'toFixed' does not exist on type 'string'. Did you mean 'fixed'?
-!!! related TS2728 /.ts/lib.es2015.core.d.ts:485:5: 'fixed' is declared here.
+!!! related TS2728 /.ts/lib.es2015.core.d.ts:487:5: 'fixed' is declared here.
     fn5 `${ (n) => n.substr(0) }`;
     
     
\ No newline at end of file
diff --git a/tests/cases/compiler/stringEndsWith.ts b/tests/cases/compiler/stringEndsWith.ts
new file mode 100644
index 0000000000000..0281bf18ad6b0
--- /dev/null
+++ b/tests/cases/compiler/stringEndsWith.ts
@@ -0,0 +1,14 @@
+// @target: es2015
+
+declare var test: string;
+declare function requirePostfix(input: `${string}test`): void;
+
+// works
+if (test.endsWith('test')) requirePostfix(test);
+if (test.endsWith('test', undefined)) requirePostfix(test);
+
+// errors
+if (test.endsWith('test', 0)) requirePostfix(test);
+if (test.endsWith('test', 1)) requirePostfix(test);
+if (test.endsWith('test', 0 as number)) requirePostfix(test);
+if (test.endsWith('test', 1 as number)) requirePostfix(test);
diff --git a/tests/cases/compiler/stringStartsWith.ts b/tests/cases/compiler/stringStartsWith.ts
new file mode 100644
index 0000000000000..cfa848a3b9199
--- /dev/null
+++ b/tests/cases/compiler/stringStartsWith.ts
@@ -0,0 +1,14 @@
+// @target: es2015
+
+declare var test: string;
+declare function requirePrefix(input: `test${string}`): void;
+
+// works
+if (test.startsWith('test')) requirePrefix(test);
+if (test.startsWith('test', undefined)) requirePrefix(test);
+if (test.startsWith('test', 0)) requirePrefix(test);
+
+// errors
+if (test.startsWith('test', 1)) requirePrefix(test);
+if (test.startsWith('test', 0 as number)) requirePrefix(test);
+if (test.startsWith('test', 1 as number)) requirePrefix(test);