Pass only the second argument in javascript

If you are OK with a change to the number of arguments that your function takes, you could pass the arguments via object properties. Then you can let the caller decide which property (or properties) to specify during the call.

The other properties can take default values through object destructuring in the function's parameter specification:

function test({a = 1, b = 2}) {
    console.log(`a = ${a}, b = ${b}`);
};

test({b:42}); // only specify what b is

With default parameter values added in ES2015, you can declare default values for the parameters, and when making the call, if you pass undefined as the first parameter, it will get the default:

function test(a = "ay", b = "bee") {
  console.log(`a = ${a}, b = ${b}`);
}
test();             // "a = ay, b = bee"
test(1);            // "a = 1, b = bee"
test(undefined, 2); // "a = ay, b = 2"
test(1, 2);         // "a = 1, b = 2"

You can do something similar yourself manually in a pre-ES2015 environment by testing for undefined:

function test(a, b) {
  if (a === undefined) {
    a = "ay";
  }
  if (b === undefined) {
    b = "bee";
  }
  console.log("a = " + a + ", b = " + b);
}
test();             // "a = ay, b = bee"
test(1);            // "a = 1, b = bee"
test(undefined, 2); // "a = ay, b = 2"
test(1, 2);         // "a = 1, b = 2"

Tags:

Javascript

Function

Arguments

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