Constructor-mode detection
Detect whether a function is being used as a constructor.
JS Puzzles · Const
new.target
new.target reveals whether the current function was called with new. It is undefined for regular calls and refers to the function during construction.
How it works
new.target is a special expression available inside function and class bodies. It is undefined for an ordinary call and identifies the constructor used for construction, allowing code to distinguish constructor mode from regular invocation.
Syntax
new.targetReturns
The expression evaluates to undefined for regular calls, or to the constructor function when called with new.
Key points
- Use
new.targetto tell whether a function was invoked as a constructor. - A direct call produces
undefined, while construction produces the called function. - This distinction supports constructor-call validation or compatibility behavior.
- A function can redirect a regular call to a constructor call when needed.
Common mistakes
- Outside constructor calls,
new.targetisundefined; do not treat it as a normal object property. - Using
new.targetoutside a function or class body causes a syntax error.
Example: Accept regular and constructor calls
function User(name) {
if (!new.target) return new User(name);
this.name = name;
}
console.log(new User("Ada").name);The function creates an instance whether it is called directly or with new.
When to use it
Call normalization
Make a constructor function work with or without explicitly writing new.
Invocation validation
Reject or handle accidental regular invocation of constructor-style functions.
Related
newconstructor