diff --git a/devs/run-tests/05strings.ts b/devs/run-tests/05strings.ts
index ed4fb2d665..943a275008 100644
--- a/devs/run-tests/05strings.ts
+++ b/devs/run-tests/05strings.ts
@@ -128,9 +128,29 @@ function testSplit() {
     isEq(sq3[2], "c,d")
 }
 
+function testReplace() {
+    const q = "a,b,c,d"
+    const sq = q.replace(",", ":")
+    isEq(sq, "a:b,c,d")
+    const q2 = "a,b,c,d"
+    const sq2 = q2.replace("c,d", "c,d,e,f")
+    isEq(sq2, "a,b,c,d,e,f")
+}
+
+function testReplaceAll() {
+    const q = "a,b,c,d"
+    const sq = q.replaceAll(",", ":")
+    isEq(sq, "a:b:c:d")
+    const q2 = "a,b,c,d,c,d"
+    const sq2 = q2.replaceAll("c,d", "c,d,e,f")
+    isEq(sq2, "a,b,c,d,e,f,c,d,e,f")
+}
+
 testStrings()
 testStringOps()
 consStringTest()
 
 testSlice()
 testSplit()
+testReplace()
+testReplaceAll()
diff --git a/packages/core/src/corelib.d.ts b/packages/core/src/corelib.d.ts
index 2a1e80e54f..4fa1ca1757 100644
--- a/packages/core/src/corelib.d.ts
+++ b/packages/core/src/corelib.d.ts
@@ -113,7 +113,19 @@ interface String {
      * If this value is not specified, the substring continues to the end of stringObj.
      */
     slice(start?: number, end?: number): string
+    /**
+     * Replace the first instance of a substring in a string, using a search string.
+     * @param searchValue The substring to search for.
+     * @param replaceValue The string containing the text to replace the matched substring.
+     */
+    replace(searchValue: string, replaceValue: string): string
 
+    /**
+     * Replace all instances of a substring in a string, using a search string.
+     * @param searchValue The substring to search for.
+     * @param replaceValue The string containing the text to replace all the matches found.
+     */
+    replaceAll(searchValue: string, replaceValue: string): string
     /** Returns the length of a String object. */
     readonly length: number
 
@@ -440,7 +452,7 @@ interface Array<T> {
     /**
      * Returns an iterable of keys in the array
      */
-    keys(): IterableIterator<number>;
+    keys(): IterableIterator<number>
 }
 
 interface ArrayConstructor {
@@ -725,10 +737,10 @@ declare var Promise: PromiseConstructor
 type Awaited<T> = T extends null | undefined
     ? T // special case for `null | undefined` when not in `--strictNullChecks` mode
     : T extends object & { then(onfulfilled: infer F, ...args: infer _): any } // `await` only unwraps object types with a callable `then`. Non-object types are not unwrapped
-    ? F extends (value: infer V, ...args: infer _) => any // if the argument to `then` is callable, extracts the first argument
-        ? Awaited<V> // recursively unwrap the value
-        : never // the argument to `then` was not callable
-    : T // non-object or non-thenable
+      ? F extends (value: infer V, ...args: infer _) => any // if the argument to `then` is callable, extracts the first argument
+          ? Awaited<V> // recursively unwrap the value
+          : never // the argument to `then` was not callable
+      : T // non-object or non-thenable
 
 // utility types
 
diff --git a/packages/core/src/string.ts b/packages/core/src/string.ts
index 1ff8a5bebf..74d9c6e477 100644
--- a/packages/core/src/string.ts
+++ b/packages/core/src/string.ts
@@ -109,6 +109,36 @@ String.prototype.split = function (
     return A
 }
 
+String.prototype.replace = function (
+    this: string,
+    searchValue: string,
+    replaceValue: string
+): string {
+    const match = this.indexOf(searchValue)
+    if (match === -1) return this
+    return (
+        this.slice(0, match) +
+        replaceValue +
+        this.slice(match + searchValue.length)
+    )
+}
+
+String.prototype.replaceAll = function (
+    this: string,
+    searchValue: string,
+    replaceValue: string
+): string {
+    let resultString = this
+    let match = this.indexOf(searchValue)
+    while (match !== -1) {
+        resultString =
+        resultString.slice(0, match) +
+        replaceValue +
+        resultString.slice(match + searchValue.length)
+        match = resultString.indexOf(searchValue, match + replaceValue.length)
+    }
+    return resultString
+}
 function splitMatch(S: string, q: number, R: string): number {
     return S.indexOf(R, q, q + 1) === q ? q + R.length : -1
 }