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JavaScript Introducing ES2015 Classes Structure of Class

Posted by joseph arias
joseph arias
9,269 Points

Default value

I don't understand why = {} was added inside the parameter. Anyone know?

 class Student {
  constructor({ name, age, interestLevel = 5 } = {}) {
    this.name = name;
    this.age = age;
    this.interestLevel = interestLevel;
  }
}

2 Answers

Steven Parker
Steven Parker
229,785 Points
Steven Parker
Steven Parker
229,785 Points

:point_right: This is the syntax for a default value.

When you see an equal sign (=) in an argument specification, it means that the argument will be given a default value if none is supplied in the call. In this case, the constructor function takes an object as the argument, and the default is an empty object, represented by the set of braces ({}). So calling this constructor with no argument is the same as calling it with an empty object.

Then, the object parameter itself has a default of 5 for the interestLevel property. So calling the constructor with no argument, or calling it with an object that has no interestLevel property, will both cause the constructed class object to have an insterestLevel property set to 5.

Tom Geraghty
Tom Geraghty
24,174 Points
Tom Geraghty
Tom Geraghty
24,174 Points

These tests might illustrate the differences:

class Student {
  constructor({ name, age, interestLevel = 5 } = {}) {
    this.name = name;
    this.age = age;
    this.interestLevel = interestLevel;
  }
} 
class StudentNoDefault {
  constructor(name, age, interestLevel = 5) {
    this.name = name;
    this.age = age;
    this.interestLevel = interestLevel;
  }
}

Now some tests:

new Student()
// Student { name: undefined, age: undefined, interestLevel: 5 }
new Student({})
// Student { name: undefined, age: undefined, interestLevel: 5 }
new Student({},{})
// Student { name: undefined, age: undefined, interestLevel: 5 }
new Student({},{},{},{},{})        //add as many parameters as you want...
// Student { name: undefined, age: undefined, interestLevel: 5 }

new StudentNoDefault()
// StudentNoDefault { name: undefined, age: undefined, interestLevel: 5 }
new StudentNoDefault({})
// StudentNoDefault { name: {}, age: undefined, interestLevel: 5 }
new StudentNoDefault({},{})
// StudentNoDefault { name: {}, age: {}, interestLevel: 5 }
new StudentNoDefault({},{},{},{},{})
// StudentNoDefault { name: {}, age: {}, interestLevel: {} }

Seeing the pattern? In the above Student class, the parameters are defined as optional by using ={} which leaves any parameter not explicitly given to be equal to undefined. If you leave out the optional ={}, then the parameters (even empty Objects {}) are assigned to the class' property (in the order they are given as parameters).

Tests ran here: https://repl.it/languages/babel