A labeled statement is any statement that is prefixed with an identifier. You can jump to this label using a break
or continue
statement nested within the labeled statement.
A labeled statement is any statement that is prefixed with an identifier. You can jump to this label using a break
or continue
statement nested within the labeled statement.
label: statement
label
Any JavaScript identifier that is not a reserved word.
statement
A JavaScript statement. break
can be used within any labeled statement, and continue
can be used within labeled looping statements.
You can use a label to identify a statement, and later refer to it using a break
or continue
statement. Note that JavaScript has no goto
statement; you can only use labels with break
or continue
.
Any break
or continue
that references label
must be contained within the statement
that's labeled by label
. Think about label
as a variable that's only available in the scope of statement
.
If a break label;
statement is encountered when executing statement
, execution of statement
terminates, and execution continues at the statement immediately following the labeled statement.
continue label;
can only be used if statement
is one of the looping statements. If a continue label;
statement is encountered when executing statement
, execution of statement
continues at the next iteration of the loop. continue;
without a label can only continue the innermost loop, while continue label;
allows continuing any given loop even when the statement is nested within other loops.
A statement can have multiple labels. In this case, the labels are all functionally equivalent.
// The first for statement is labeled "loop1" loop1: for (let i = 0; i < 3; i++) { // The second for statement is labeled "loop2" loop2: for (let j = 0; j < 3; j++) { if (i === 1 && j === 1) { continue loop1; } console.log(`i = ${i}, j = ${j}`); } } // Logs: // i = 0, j = 0 // i = 0, j = 1 // i = 0, j = 2 // i = 1, j = 0 // i = 2, j = 0 // i = 2, j = 1 // i = 2, j = 2
Notice how it skips both "i = 1, j = 1" and "i = 1, j = 2".
let i, j; // The first for statement is labeled "loop1" loop1: for (i = 0; i < 3; i++) { // The second for statement is labeled "loop2" loop2: for (j = 0; j < 3; j++) { if (i === 1 && j === 1) { break loop1; } console.log(`i = ${i}, j = ${j}`); } } // Logs: // i = 0, j = 0 // i = 0, j = 1 // i = 0, j = 2 // i = 1, j = 0
Notice the difference with the previous continue
example: when break loop1
is encountered, the execution of the outer loop is terminated, so there are no further logs beyond "i = 1, j = 0"; when continue loop1
is encountered, the execution of the outer loop continues at the next iteration, so only "i = 1, j = 1" and "i = 1, j = 2" are skipped.
Given an array of items and an array of tests, this example counts the number of items that pass all the tests.
// Numbers from 1 to 100 const items = Array.from({ length: 100 }, (_, i) => i + 1); const tests = [ { pass: (item) => item % 2 === 0 }, { pass: (item) => item % 3 === 0 }, { pass: (item) => item % 5 === 0 }, ]; let itemsPassed = 0; itemIteration: for (const item of items) { for (const test of tests) { if (!test.pass(item)) { continue itemIteration; } } itemsPassed++; }
Note how the continue itemIteration;
statement skips the rest of the tests for the current item as well as the statement that updates the itemsPassed
counter, and continues with the next item. If you don't use a label, you would need to use a boolean flag instead.
// Numbers from 1 to 100 const items = Array.from({ length: 100 }, (_, i) => i + 1); const tests = [ { pass: (item) => item % 2 === 0 }, { pass: (item) => item % 3 === 0 }, { pass: (item) => item % 5 === 0 }, ]; let itemsPassed = 0; for (const item of items) { let passed = true; for (const test of tests) { if (!test.pass(item)) { passed = false; break; } } if (passed) { itemsPassed++; } }
Given an array of items and an array of tests, this example determines whether all items pass all tests.
// Numbers from 1 to 100 const items = Array.from({ length: 100 }, (_, i) => i + 1); const tests = [ { pass: (item) => item % 2 === 0 }, { pass: (item) => item % 3 === 0 }, { pass: (item) => item % 5 === 0 }, ]; let allPass = true; itemIteration: for (const item of items) { for (const test of tests) { if (!test.pass(item)) { allPass = false; break itemIteration; } } }
Again, if you don't use a label, you would need to use a boolean flag instead.
// Numbers from 1 to 100 const items = Array.from({ length: 100 }, (_, i) => i + 1); const tests = [ { pass: (item) => item % 2 === 0 }, { pass: (item) => item % 3 === 0 }, { pass: (item) => item % 5 === 0 }, ]; let allPass = true; for (const item of items) { let passed = true; for (const test of tests) { if (!test.pass(item)) { passed = false; break; } } if (!passed) { allPass = false; break; } }
You can label statements other than loops, such as simple blocks, but only break
statements can reference non-loop labels.
foo: { console.log("face"); break foo; console.log("this will not be executed"); } console.log("swap"); // Logs: // "face" // "swap"
Labels can only be applied to statements, not declarations. There is a legacy grammar that allows function declarations to be labeled in non-strict code:
L: function F() {}
In strict mode code, however, this will throw a SyntaxError
:
"use strict"; L: function F() {} // SyntaxError: functions cannot be labelled
Non-plain functions, such as generator functions and async functions can neither be labeled in strict code, nor in non-strict code:
L: function* F() {} // SyntaxError: generator functions cannot be labelled
The labeled function declaration syntax is deprecated and you should not use it, even in non-strict code. You cannot actually jump to this label within the function body.
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https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/label