undefined

Undefined is a type that consists of a single value undefined. It is used to indicate that a variable has not been assigned a value.

If a variable is initialized using let or var and no value is assigned to it, JavaScript automatically sets the value to undefined.

JavaScript automatically sets the value of undefined to a new variable:

        
          
          let nameconsole.log(name)// undefined
          let name

console.log(name)
// undefined

        
        
          
            
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JavaScript also automatically sets the value of undefined in a function argument if no value is passed during the call:

        
          
          function hello(name) {  console.log('Hello, ' + name)}hello('Vitya')// Hello, Vityahello()// Hello, undefined
          function hello(name) {
  console.log('Hello, ' + name)
}

hello('Vitya')
// Hello, Vitya
hello()
// Hello, undefined

        
        
          
            
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Manually setting undefined is used to indicate an unknown value:

        
          
          const person = {  name: 'Pyotr',  lastName: 'Romanov',  age: undefined}
          const person = {
  name: 'Pyotr',
  lastName: 'Romanov',
  age: undefined
}

        
        
          
            
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But be careful when setting undefined to object properties. If the age property is removed from the person object, accessing the nonexistent property will also return undefined.

undefined indicates that the value has not yet been set or is unknown for some reason. In this context, it is used by the JavaScript language itself.

There exists a similar primitive value null. It indicates the intentional absence of an object value. undefined and null can easily be confused because they are closely related concepts. The difference is that null indicates the intentional absence of an object value, while undefined means that the variable has no value assigned, but this variable is not an object.

        
          
          // The variable person is intended for an object, but the structure of the object is not yet knownlet person = null// We've decided on the structure of the objectperson = {  name: 'Pyotr',  age: 25,  // The field profession — string. Pyotr has not defined it yet, hence undefined  profession: undefined}
          // The variable person is intended for an object, but the structure of the object is not yet known
let person = null

// We've decided on the structure of the object
person = {
  name: 'Pyotr',
  age: 25,

  // The field profession — string. Pyotr has not defined it yet, hence undefined
  profession: undefined
}

        
        
          
            
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Another interesting example that clearly shows what "absence of value" means:

        
          
          function toLog(a, b = a) {  console.log(a, b)}toLog(1, undefined) // 1 1toLog(1, null) // 1 nulltoLog(1) // 1 1
          function toLog(a, b = a) {
  console.log(a, b)
}

toLog(1, undefined) // 1 1
toLog(1, null) // 1 null
toLog(1) // 1 1

        
        
          
            
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When the argument is not passed, JavaScript implicitly substitutes undefined. The code in the example appears as if we passed an argument. In reality, this notation is no different from the case where the argument is not passed at all and the default value is used.

        
          
          const data = {  users: [] // Array of users  success: true // Server response status}
          const data = {
  users: [] // Array of users
  success: true // Server response status
}

        
        
          
            
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          let data = null// Some logicdata = await getUsers() // Awaiting server response in the form of an object
          let data = null

// Some logic

data = await getUsers() // Awaiting server response in the form of an object

        
        
          
            
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