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Pass a second parameter to the forEach callback: the first parameter is the current item, and the second is its zero-based index.
const items = ["apple", "banana", "cherry"];
items.forEach((item, index) => {
console.log(index, item);
});This logs 0 apple, 1 banana, and 2 cherry. TypeScript documents the callback parameters in this order: array element, index, then the containing array (TypeScript Handbook: Type Compatibility).
Use the second callback parameter
Name the second parameter index or i. TypeScript infers its type for an array callback, so you usually do not need to annotate it.
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const items = ["apple", "banana", "cherry"];
items.forEach((item, index) => {
console.log(`${index}: ${item}`);
});The callback receives the current element first and its zero-based position second. Its optional third parameter is the array being traversed:
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items.forEach((item, index, array) => {
console.log(array[index] === item);
});If you only need the item, you can omit the other parameters; callback parameters you do not use can be ignored.
When to use a different loop
Use a counted for loop for explicit control
If you need to manage the index yourself or control the loop flow, use a counted loop:
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for (let index = 0; index < items.length; index++) {
const item = items[index];
console.log(index, item);
}With TypeScript’s noUncheckedIndexedAccess option enabled, an expression such as items[index] is checked as potentially undefined. The callback form passes the current element directly. See the TypeScript 4.1 release notes for the option.
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A for...of loop yields values, not their indexes. It is suitable when the index is unnecessary.
Do not use for...in for an array index
A for...in loop visits keys. For arrays, those keys are strings, so it is not the same as receiving a numeric position in a forEach callback. TypeScript describes for...in as a way to inspect object properties; see Iterators and Generators.
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