JavaScript String replace()

 const message = "ball bat"; 
// replace the first b with c let result = message.replace('b', 'c');
 console.log(result); // Output: call bat

replace() Syntax

The syntax of replace() is:

 str.replace(pattern, replacement)

Here, str is a string.

replace() Parameter

The replace() method takes in:

• pattern - either a string or a regex that is to be replaced
• replacement - the pattern is replaced with this replacement (can be either a string or a function)

relace() Return Value

• The replace() method returns a new string with the specified pattern replaced.

Example 1: Replace the first occurrence

 const text = "Java is awesome. Java is fun." // passing a string as the first parameter 
let pattern = "Java"; let new_text = text.replace(pattern, "JavaScript");
 console.log(new_text); // passing a regex as the first parameter 
pattern = /Java/; new_text = text.replace(pattern, "JavaScript");
 console.log(new_text);

Output

 JavaScript is awesome. Java is fun. JavaScript is awesome. Java is fun.

In both replace() methods, the first occurrence of Java is replaced with JavaScript.

Example 2: Replace all occurrences

To replace all occurrences of the pattern, you need to use a regex with a g switch (global search). For example, /Java/g instead of /Java/.

 const text = "Java is awesome. Java is fun." // notice the g switch in the regex pattern 
const pattern = /Java/g; const new_text = text.replace(pattern, "JavaScript");
 console.log(new_text);

Output

 JavaScript is awesome. JavaScript is fun.

Here, the replace() method replaces both occurrences of Java with JavaScript.

Replace Without Considering Uppercase/Lowercase

The replace() method is case sensitive. To perform the case-insensitive replacement, you need to use a regex with an i switch (case-insensitive search).

Example 3: Case-Insensitive Replacement

 const text = "javaSCRIPT JavaScript" // the first occurrence of javascript is replaced let pattern = /javascript/i; // case-insensitive search let new_text = text.replace(pattern, "JS"); console.log(new_text) // JS JavaScript // all occurrences of javascript is replaced pattern = /javascript/gi; // case-insensitive and global search new_text = text.replace(pattern, "JS"); console.log(new_text) // JS JS

Output

 JS JavaScript JS JS

Example 4: Passing Function as a Replacement

You can also pass a function (instead of a string) as the second parameter to the replace() method.

 const text = "Random digit: 3" // generate a random digit between 0 and 9 function generateRandomDigit() { return Math.floor(Math.random() * 10) } // regex to match a digit 
const pattern = /\d/; const new_text = text.replace(pattern, generateRandomDigit);
 console.log(new_text)

Output

 Random digit: 8

You may get different output when you run this program. It's because the first digit in text is replaced with a random digit between 0 and 9.

Recommended Reading: JavaScript String replaceAll()