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JavaScript Introducing ES2015 Objects and New Collection Types Map

Kayc M
Kayc M
12,593 Points
Posted by Kayc M
Kayc M
12,593 Points

Can't log map using template literals, why?

let joe = { name: 'joe', age: 20},
ann = {name: 'ann', age: 20 };

let classroom = new Map()

classroom.set('joe', joe)
classroom.set('ann', ann)

console.log('print map: ', classroom)
output:
// print map:  Map(2) {
  //   'joe' => { name: 'joe', age: 20 },
  //   'ann' => { name: 'ann', age: 20 }
  // }


  console.log(`print map: ${classroom}`)
  output:
  // print map: [object Map]

2 Answers

Steven Parker
Steven Parker
231,261 Points
Steven Parker
Steven Parker
231,261 Points

Modern browsers may evaluate the arguments to the "log" method and invoke a special conversion designed to display that type of object, but the template string relies on the object's own string conversion method.

So:

console.log(`print map: ${classroom}`);              // is equivalent to...
console.log('print map: ' + classroom.toString());
Kayc M
Kayc M
12,593 Points
Kayc M
Kayc M
12,593 Points

But why does concatenating without "toString" have the same result 

console.log('print map: ' + classroom);
output:
  // print map: [object Map]
Steven Parker
Steven Parker
231,261 Points
Steven Parker
Steven Parker
231,261 Points

Concatenating a non-string onto a string also invokes the built-in (toString) conversion. What the browser does for you when it is a separate argument is a new feature above and beyond the language itself, and not long ago in previous versions would have looked the same as the others.