Labeled Statement

Documentation Index

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Syntax

labelIdentifier: statement

• labelIdentifier: Any valid JavaScript/TypeScript identifier that is not a reserved word.
• statement: The code block or loop being labeled.

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Control Flow Mechanics

1. break labelIdentifier; Terminates the execution of the labeled statement and transfers control to the statement immediately following it. While break typically only applies to loops and switch statements, a labeled break can be applied to any labeled block statement.

outerBlock: {
    console.log("Entering block");
    if (true) {
        break outerBlock; // Exits 'outerBlock' immediately
    }
    console.log("Unreachable code");
}

2. continue labelIdentifier; Terminates the current iteration of the labeled loop and evaluates the loop’s condition for the next iteration. Unlike break, a labeled continue can only be applied to iterative statements (for, while, do...while).

outerLoop: for (let i = 0; i < 5; i++) {
    for (let j = 0; j < 5; j++) {
        if (j === 2) {
            continue outerLoop; // Skips remaining inner loop, advances 'i'
        }
    }
}

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Technical Constraints and Scope

• Namespace Isolation: Labels reside in a dedicated namespace. A label identifier can share the exact same name as a variable or function in the same scope without causing a naming collision.

const foo = 10;
foo: for (let i = 0; i < foo; i++) { // Valid TS syntax
    break foo;
}

• Lexical Boundaries: The scope of a label is strictly limited to the statement it prefixes. Control flow cannot cross function boundaries; a break or continue cannot reference a label situated outside of its immediate enclosing function, including arrow functions or callbacks.

loopLabel: for (let i = 0; i < 3; i++) {
    setTimeout(() => {
        // break loopLabel; // TS Error: Undefined label 'loopLabel'
    });
}

• Duplicate Identifiers: TypeScript throws a syntax error if overlapping (nested) statements share the same label identifier. However, identical labels are permitted if their scopes are entirely disjointed.
• Strict Mode Restrictions: Because TypeScript modules execute in strict mode by default, certain identifiers like let, static, yield, and await are strictly prohibited from being used as label names.