Replace

In Node.js, a string can be changed in many ways. With replace, we swap a part of a string with another substring. The result is copied into a new string.

For optimization, replace is an important method to focus on. Often a replace() call is used in a function's fast path. We can apply measures to optimize (or eliminate) replace.

Example

With this string method we replace the first instance of a substring with a replacement substring. The searching begins from the left part of the string.

var animals = "bird, cat, bird";

// Replace first instance of bird with frog.
var result = animals.replace("bird", "frog");
console.log("REPLACE RESULT: " + result);
REPLACE RESULT: frog, cat, bird

Original left alone

When we call replace() the original string is left alone. So a string is not mutated in-place. A new copy is created—we can use both forms in our program.

var initial = "abc def ghi";

// Replace one part of the string.
var result = initial.replace("abc", "xyz");

// The initial string is left unchanged.
console.log("BEFORE:        " + initial);
console.log("AFTER REPLACE: " + result);
BEFORE:        abc def ghi
AFTER REPLACE: xyz def ghi

Regex

Sometimes a more complex replacement method is needed. With Regex we can replace a pattern. All matching patterns can be replaced with a "g" modifier.

var data = "cat cap city car";
console.log("BEFORE:          " + data);

// Replace all matching patterns with a string.
// ... Remove the g to only replace the first match.
var result = data.replace(/ca\w/g, "test");
console.log("PATTERN REPLACE: " + result);
BEFORE:          cat cap city car
PATTERN REPLACE: test test city test

Replace URL characters

For URLs, we often need to handle special characters like "#" and spaces. We can use replace() for these, but encodeURIComponent is simpler and faster.

Replace notes

With replace we can use regular expression and string arguments. We investigated the performance of these methods.