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Put break; inside a loop body to stop that loop as soon as a condition is met. Execution then continues with the first statement after the loop. TypeScript uses JavaScript’s loop-control behavior; its type annotations do not change what break does at runtime.
Stop a loop when a condition is met
Usually, place break; inside an if statement in the loop body. When the condition is true, the loop ends immediately. Any remaining statements in that iteration are skipped.
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const values = [3, 7, 12, 18];
for (const value of values) {
if (value > 10) {
break;
}
console.log(value);
}
console.log("Loop finished");This prints 3, 7, and then Loop finished. When the loop reaches 12, break skips the rest of that iteration and exits the loop; it does not print 12 or visit 18. The TypeScript Handbook describes break as a way to exit a loop in its discussion of variable declarations: Variable Declaration.
Use a loop to stop at the first match
Save the result before breaking if you need to use it after the loop.
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const names = ["Ada", "Grace", "Linus"];
let match: string | undefined;
for (const name of names) {
if (name.startsWith("G")) {
match = name;
break;
}
}
console.log(match); // "Grace"The loop stops at the first name beginning with G. If no item matches, match remains undefined.
What happens in nested loops?
An unlabeled break exits only the innermost enclosing loop. The outer loop continues with its next iteration.
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for (const row of [1, 2]) {
for (const column of [1, 2, 3]) {
if (column === 2) {
break;
}
console.log(row, column);
}
}
// Prints: 1 1, then 2 1Here, each inner loop stops when column reaches 2, but the outer loop still advances to the next row. To leave multiple levels, you can label a loop and use a labeled break, or put the search in a function and return when you find the result. A labeled break exits the labeled statement and the loops nested inside it:
search:
for (const row of [1, 2]) {
for (const column of [1, 2, 3]) {
if (row === 2 && column === 2) {
break search;
}
console.log(row, column);
}
}Does break work in for...of?
Yes. A for...of loop can be exited with break just like a classic for loop. Use for...of when the loop needs values from an iterable, such as array elements. Use for...in when you need an object’s property keys; it does not serve as a substitute for iterating over array values.
The TypeScript Handbook explains the value-versus-key distinction in Iterators and Generators. For older JavaScript targets, emitted iteration behavior can depend on the target and downlevelIteration settings. See the downlevelIteration TSConfig reference.
Choose the loop form that fits
- Classic
for: Use it when the index or iteration count matters, such as scanning an array by position. for...of: Use it when you need values from an iterable and do not need to manage an index yourself.for...in: Use it when you need property keys, not array values.
Common mistakes and compiler warnings
- Using
breakoutside a loop orswitch: It must appear in a breakable construct. - Expecting an inner-loop break to stop every loop: A plain
breakexits only its innermost enclosing loop. - Leaving required work after the break: Statements later in the same iteration are skipped. Put any work that must happen before the exit ahead of
break. - Breaking unconditionally and then writing more code in the same block: A compiler may flag the following code as unreachable. TypeScript 1.8 release notes describe the
allowUnreachableCodeoption for disabling unreachable-code reporting: TypeScript 1.8 release notes. - Assuming TypeScript types alter loop behavior: Types help check your code, but loop control follows JavaScript semantics. The Handbook describes TypeScript as an extension of JavaScript with a type system: TypeScript for JavaScript Programmers.
Using break with a using declaration
If a using variable is declared as part of a for statement, leaving the loop with break disposes of that variable according to the TypeScript Handbook’s Variable Declaration guidance. This matters when the variable represents a resource whose lifetime is managed by using.
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