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JavaScript | handler.set() Method

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  • Difficulty Level : Hard
  • Last Updated : 29 Sep, 2021

The handler.set() method in JavaScript is a trap for setting a property value. This method returns a boolean value.
Syntax: 
 

const p = new Proxy(target, {
  set: function(target, property, value, receiver) {
  }
});

Parameters: This method accept four parameters as mentioned above and described below: 
 

  • target: This parameter holds the target object.
  • property: This parameter holds the name or Symbol of the property.
  • value: This parameter holds the new value of the property.
  • receiver: This parameter holds the object to which the assignment was originally directed.

Return value: This method always return a boolean value.
Below examples illustrate the handler.set() Method in JavaScript:
Example 1: 
 

javascript




function gfg() {
  this.users = "Millions";
}
 
const handler1 = {
  set(obj, prop, value) {
    if ((prop === 'users') && ((value % 2) !== 0)) {
      console.log('GEEKSFORGEEKS : Computer Science Portal');
    } else {
      return Reflect.set(...arguments);
    }
  }
};
 
const gfg1 = new gfg();
const proxy1 = new Proxy(gfg1, handler1);
proxy1.users = 1;
 
console.log(proxy1.users);

Output: 
 

"GEEKSFORGEEKS : Computer Science Portal"
"Millions"

Example 2: 
 

javascript




const p = new Proxy({}, {
  set: function(target, prop, value, receiver) {
    target[prop] = value;
    console.log('property set: ' + prop + ' = ' + value);
    return true;
  }
})
 
console.log('a' in p); 
 
p.a = 10;            
console.log('a' in p); 
console.log(p.a);
 
var x = { foo: 1 }; 
var proxy = new Proxy(x, { 
  set: function(target, name, value, proxy) {   
target[name] = value+" --> "+ value.toUpperCase(); 
  
}); 
proxy.foo = 'geeksforgeeks'
console.log(x.foo);

Output: 
 

false
"property set: a = 10"
true
10
"geeksforgeeks --> GEEKSFORGEEKS"

Supported Browsers: The browsers supported by handler.set() method are listed below. 
 

  • Google Chrome 49 and above
  • Edge 12 and above
  • Firefox 18 and above
  • Opera 36 and above
  • Safari 10 and above

 


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