Using map in the function is mutating the main array

问题

I'm new to JS and trying to use map array.prototype.map() function in my code. I need to add one property to the array's each element without changing the main object structure. I have this array of objects

const nav = [
  {
    name: 'Home',
    title: 'Dashboard',
    icon: 'home',
  },
  {
    name: 'About',
    icon: 'layout-auto',
    title: 'About',
  },
  {
    name: 'Applications',
    icon: 'applications',
    title: 'Applications',
  },
];

after mapping through this array it also chaging main nav array's objects.

const mappedNav = (arr) => {
  let newArr = arr.map(nav => {
    nav.title = 'test';
    return element;
  });
}

mapNav(nav);

console.table(nav);

How can I add properties to the new array's objects wihout mutating them?

回答1:

You can use the spread syntax (...). Using spread inside of an object literal will copy the (own enumerable) keys of the object into the newly created object literal ({}). This way you're mapping each object to a new object, which has the same keys and values as the current object, along with an overwritten/new title attribute.

const nav = [ { name: 'Home', title: 'Dashboard', icon: 'home', }, { name: 'About', icon: 'layout-auto', title: 'About', }, { name: 'Applications', icon: 'applications', title: 'Applications', } ];

const res = nav.map(obj => {
  return {...obj, title: "test"};
});
console.log(res);

Note that object spread is ES9, if you want ES6, you can use Object.assign() instead, which will merge the properties of one (or more) objects into each other.

const nav = [ { name: 'Home', title: 'Dashboard', icon: 'home', }, { name: 'About', icon: 'layout-auto', title: 'About', }, { name: 'Applications', icon: 'applications', title: 'Applications', } ];

const res = nav.map(obj => {
  return Object.assign({}, obj, {title: "test"});
// merge -------------(1^)-(2^)--(3^)
// (1) - the new object to return (used to make a shallow copy)
// (2) - merge obj (2) into (1)
// (3) - merge keys from {title: "test"} into the result of (2) above 
});
console.log(res);

回答2:

map() function with spread operator can achieve this.

    const nav = [
      {
        name: 'Home',
        title: 'Dashboard',
        icon: 'home',
      },
      {
        name: 'About',
        icon: 'layout-auto',
        title: 'About',
      },
      {
        name: 'Applications',
        icon: 'applications',
        title: 'Applications',
      },
    ].map(v => ({...v, title: "test"}));
    
    console.log(nav)

来源：https://stackoverflow.com/questions/63915352/using-map-in-the-function-is-mutating-the-main-array