Run a function once in JavaScript

Wraps a function so it runs the first time and returns that result forever after.

function once(fn) {
  let called = false;
  let result;
  return function (...args) {
    if (!called) {
      called = true;
      result = fn.apply(this, args);
    }
    return result;
  };
}

Keywords and builtins used here

• function
• if
• let
• return
• this

The run, in numbers

Lines

11

Characters to type

162

Tokens

49

Three-star pace

100 tpm

At the three-star pace of 100 tokens a minute, this run takes about 29 seconds.

Type this snippet

Step 2 of 4 in Closures, step 2 of 16 in Functions & patterns.

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