Delete is an operation that is not used very often in projects, but in some cases, the result of delete is not quite what I expected.

To delete a property of an object:

grammar

delete expression

Where expression is evaluated as a reference to a property

const Employee = {
  firstname: 'John',
  lastname: 'Doe'
}
console.log(Employee.firstname); // > "John"
delete Employee.firstname;
console.log(Employee.firstname); // > undefined
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return

  • Returns true

  • If the property is not configurable, non-strict mode returns false, and strict mode throws a syntax error

      var Employee = {};
  Object.defineProperty(Employee, 'name', {configurable: false});
  console.log(delete Employee.name);  //  > false
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  • If the property does not exist, it still returns true, but the delete statement has no effect. Right

      var Employee = {
    age: 28,
    name: 'abc',
    designation: 'developer'
  }
  console.log(delete Employee.name);   // > true
  console.log(delete Employee.salary); // > true
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Pay attention to

  • Delete applies only to the object’s own properties and cannot delete properties on the stereotype chain

      function Foo() {
    this.bar = 10;
  }
  
  Foo.prototype.bar = 42;
  var foo = new Foo();
  delete foo.bar;           // > true
  console.log(foo.bar);  // > 42
  delete Foo.prototype.bar; 
  console.log(foo.bar); // > undefined
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  • Properties declared with var are not configurable and cannot be deleted by delete

    var nameOther = 'XYZ'; / / window. NameOther = "XYZ Object. GetOwnPropertyDescriptor (Windows, 'nameOther'); // > Object {value: "XYZ", // writable: true, // enumerable: true, // configurable: false} delete nameOther; // > falseCopy the code

  • Global scope properties created without var, let, or const can be deleted by delete

    adminName = 'xyz'; / / window, adminName = "xyz Object. GetOwnPropertyDescriptor (window, 'adminName') / / > Object {value: 'xyz', // writable: true, // Enumerable: true, // signals: true} Delete adminName; // > trueCopy the code

  • Delete cannot delete a function in global scope. It cannot delete any property declared as let const in scope

      function f() {
    var z = 44;
    console.log(delete z); 
  }
  f() // >false
  
  delete f // >false
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  • Delete cannot delete built-in static properties

      delete Math.PI // > false
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  • When an array element is deleted, the array length is not affected, but the deleted element no longer belongs to the array, and the delete operation does not directly free memory.

    var trees = ["redwood","bay","cedar","oak","maple"]; delete trees[3]; if (3 in trees) { console.log('will not be printed') } console.log(trees) // > (5)["redwood","bay","cedar", The empty, "maple"]Copy the code