JS Type Conversion

JavaScript tries its best to execute any expression by dynamically converting data types.

When a value of one type is expected, you may provide a value of any type and JavaScript will attempt to convert the provided value into the desired type.

In each of these expressions, a boolean is expected but a number or string is used instead.

true == 1 // true

if (-999) {
    console.log('of course');
} // 'of course' --> -999 is truthy

if ('hello') {
    console.log('really?');
} // 'really?' --> "hello" is truthy

true == 0 // false
true == '' // false

Above, true is equivalent to an un-empty string ('hello'), but it is not equivalent to an empty string ('').

In JavaScript these variables are said to have truthy and falsy values, which correspond to true or false.

• truthy

  • Non-empty strings
  • Non-zero numbers
  • Objects
  • Arrays
  • true

• falsy

  • Empty strings
  • Zero (0)
  • false
  • NaN
  • null
  • undefined

Here is an example of how truthy and falsy values can be used with JavaScript:

let name; // undefined
let greeting; // undefined

if (name) {
  /*
    name is undefined.

    undefined is a 'falsy' value that will evaluate
    to false in this conditional.

    This 'if' block will be skipped and the code will
    assign a value to greeting in the 'else' block
  */
  greeting = 'Hello ' + name;
} else {
  greeting = 'Hello World!';
}

console.log(greeting); // 'Hello World!'

truthy and falsy can be very useful when validating input values before they reach your database:

let fname;
let lname = 'Johnson';

if (fname && lname) {
  // Do some server logic to store data
} else {
  throw new Error('Name fields must not be blank');
}

/*
Error: Name fields must not be blank
    at repl:4:7
    at REPLServer.defaultEval (repl.js:164:27)
    at bound (domain.js:280:14)
    at REPLServer.runBound [as eval] (domain.js:293:12)
    at REPLServer.<anonymous> (repl.js:393:12)
    at emitOne (events.js:82:20)
    at REPLServer.emit (events.js:169:7)
    at REPLServer.Interface._onLine (readline.js:210:10)
    at REPLServer.Interface._line (readline.js:549:8)
    at REPLServer.Interface._ttyWrite (readline.js:826:14)
*/

---

Prev -- Up -- Next