Get arity function

Question

In Javascript, how to determine the number of formal parameters defined for a function?

Note that this is not the arguments parameter when calling the function, but the number of named arguments with which the function was defined.

 function zero() { // Should return 0 } function one(x) { // Should return 1 } function two(x, y) { // Should return 2 } 

Answer 1

 > zero.length 0 > one.length 1 > two.length 2 

Source

A function can define its own arity (length) as follows:

 // For IE, and ES5 strict mode (named function) function foo(x, y, z) { return foo.length; // Will return 3 } // Otherwise function bar(x, y) { return arguments.callee.length; // Will return 2 } 

Answer 2

The arity function is stored in the .length property.

 function zero() { return arguments.callee.length; } function one(x) { return arguments.callee.length; } function two(x, y) { return arguments.callee.length; } > console.log("zero="+zero() + " one="+one() + " two="+two()) zero=0 one=1 two=2 

Answer 3

As described in other answers, the length property reports this. So zero.length will be 0, one.length will be 1, and two.length will be 2.

As in ES2015, we have two wrinkles:

• Functions can have a “rest” parameter at the end of the parameter list, which collects any arguments given at or after this position into a real array (as opposed to the arguments pseudo-array)
• Function parameters may have default values.

The "rest" parameter is not taken into account when determining the arity of a function:

 function stillOne(a, ...rest) { } console.log(stillOne.length); // 1 

Similarly, a parameter with a default argument does not add to arity and actually prevents any others following it from adding to it, even if they do not have explicit default values ​​(it is assumed that they have a default value of undefined ):

 function oneAgain(a, b = 42) { } console.log(oneAgain.length); // 1 function oneYetAgain(a, b = 42, c) { } console.log(oneYetAgain.length); // 1 

Answer 4

The arity of a function is the number of parameters that the function contains. It can be achieved by calling the length property.

Example:

 function add(num1,num2){} console.log(add.length); // --> 2 function add(num1,num2,num3){} console.log(add.length); // --> 3 

Note: the number of parameters passed in the function call does not affect the functionality.

Answer 5

The arity property used to return the number of arguments expected by the function, however, it no longer exists and is replaced by the Function.prototype.length property.