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javascriptreadonly-attribute

What is the definition of a read-only property in JavaScript? What does it mean?

I am a little confused on what a read-only property means? I know that classList is a read only property by MDN's definition but what does that exactly mean?

Solution

  • A read-only property means it cannot be overwritten or assigned to. Any such assignment will silently do nothing in non-strict mode. E.g.:

    var obj = {};
Object.defineProperty(obj, 'property', {value: 123, writeable: false})


// Assign 456 to obj.property using the . syntax, but it's still 123
obj.property = 456;
console.log(obj.property);

// Assign 789 to obj.property using the [] syntax, but it's still 123
obj['property'] = 789;
console.log(obj['property']);

    Or just error out with a TypeError in strict mide:

    'use strict';

var obj = {};
Object.defineProperty(obj, 'property', {value: 123, writeable: false})


// Assign 456 to obj.property in strict mode will result in a TypeError
obj.property = 456;
console.log(obj.property);