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const names: string[] = ["Ada", "Grace", "Linus"];
for (const name of names) {
console.log(name);
}TypeScript infers name as the array’s element type. Choose an indexed loop when you need positions, or an array method when you want to transform or select elements.
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Choose the loop that matches the task
| What you need | Use | Why |
|---|---|---|
| Read each value, with ordinary loop control | for...of |
Provides each element directly; supports break and continue. |
| Use an element’s position or assign to an array slot | Indexed for loop |
Gives access to the index as well as the element. |
| Run a callback for each element, without needing early exit | forEach |
Concise callback-based traversal. |
| Create a transformed array | map |
Returns a new array containing callback results. |
| Keep elements that meet a condition | filter |
Returns a new array containing selected elements. |
Loop through values with for...of
Arrays are iterable, so for...of visits their values in order. Use it when you need each element but not its numeric position:
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for (const score of scores) {
console.log(score);
}Because this is a loop statement, you can skip an item with continue or stop the loop with break:
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for (const score of scores) {
if (score < 80) continue;
if (score >= 90) break;
console.log(score);
}The TypeScript handbook describes an iterable as an object with an implementation of Symbol.iterator. Its Iterators and Generators handbook page also distinguishes for...of, which yields values, from for...in, which yields property keys.
Use an indexed loop when you need positions
A numeric loop is useful when the index matters, such as when displaying numbered entries or updating a slot:
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const names: string[] = ["Ada", "Grace", "Linus"];
for (let i = 0; i < names.length; i++) {
console.log(i, names[i]);
}With noUncheckedIndexedAccess enabled, TypeScript treats an indexed lookup such as names[i] as potentially undefined. A bounds check in the loop condition does not necessarily remove that possibility from the type. The TypeScript 4.1 release notes explain this behavior and suggest using for...of or forEach when the index is not needed.
Use forEach for callback-based traversal
When each element needs an action but the operation does not need to stop the loop early, forEach is a callback alternative:
names.forEach((name, index) => {
console.log(index, name);
});The callback receives the value and can also receive its index. Unlike a loop statement, forEach does not offer the same straightforward break or continue control; use for...of if you need to exit early.
Use map to transform or filter to select
Build a new array with map
Use map when the result should contain one transformed value for each input element:
const nameLengths = names.map(name => name.length);This returns a new array; it is not just a loop for performing an action without using its results.
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Keep matching values with filter
Use filter when you want a new array containing only elements that satisfy a condition:
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const passingScores = scores.filter(score => score >= 80);In TypeScript 5.5 and later, qualifying filter callbacks can benefit from inferred type predicates. For example, filtering out undefined can narrow the resulting element type. This inference applies when TypeScript can establish the predicate from the callback; it is not a guarantee for every conditional expression. See the TypeScript 5.5 release notes.
Why for...in is usually the wrong array loop
for...in enumerates property keys, so with an array it gives keys such as "0", "1", and "2"—not the element values. Use it only when you intentionally need to enumerate keys; for array values, prefer for...of. The distinction and examples are in the TypeScript iterators documentation.
Check target and runtime support for older environments
for...of relies on the iterable protocol. The TypeScript handbook notes that when targeting ES5, iterators are permitted only for arrays unless downlevelIteration is enabled. That compiler option changes emitted code to use a helper that checks for native or polyfilled Symbol.iterator, with fallback behavior if it is unavailable. Consult the TSConfig reference for downlevelIteration alongside the handbook’s iterators guidance.
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For a normal array in a modern runtime, for...of is a clear default. If the project targets an older environment or loops over a non-array iterable, verify the TypeScript target, whether downlevelIteration is enabled, and whether the runtime provides Symbol.iterator. TypeScript 5.6 documents the protocol-related Iterable, Iterator, and IteratorObject types in its release notes.
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