Javascript : optional parameters in function

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Is there a better way to do optional function parameters in JavaScript? [duplicate] 29 answers

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Any unfilled argument will be undefined .

concatenate(a, c) is equivalent to concatenate(a, b) . You cannot pass the third parameter without passing the second; but you can pass undefined (or null , I suppose) explicitly: concatenate(a, undefined, c) .

In the function, you can check for undefined and replace with a default value.

Alternately, you can use an object argument to imitate keyword arguments: concatenate({a: a, c: c}) .

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只需使用arguments数组对象：

function concatenate() {
  var result = '';
  for (var i = 0; i < arguments.length; i++) {
    result += arguments[i];
  }
  return result;
}

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Use the ES6 rest parameters syntax to get an array of arguments. Then simply join its items to retrieve the concatenated string.

concatenate(a);
concatenate(a, b);
concatenate(a, c);

function concatenate(...args){
  // for old browsers
  // `...args` is an equivalent of `[].slice.call(arguments);`
  return args.join('');
}

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