What does "in" mean in programming?

in is one of the most common terms in software development. This guide explains what it means, how it is used in practice, and why it matters, with real examples from the developer world.

Definition

A keyword used in for...in loops to iterate over the enumerable keys of an object, and as an operator to check whether a property exists in an object.

Example

"if ('name' in user) { console.log(user.name); }"

Related terms

for
operator
object

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