Implicit type coercion with objects

Question

var a = {};
a + 1 // return "[object Object]1"

I know why this happened. Object toPrimitive happened and after a.toString return [object Object] and merge with number But why when I type code like this

{} + 1 // return 1

Object not converted string?

Also why object toPrimitive hint Number return 0 When object convert to number this is look valueOf function and why valueOf return 0?

[What is the explanation for these bizarre JavaScript behaviours mentioned in the 'Wat' talk for CodeMash 2012?](https://stackoverflow.com/questions/9032856) — adiga, Jul 28 '19 at 11:47

score 4 · Accepted Answer · answered Jul 28 '19 at 11:37

4

First one is an object and you're adding object with number

var a = {};
a + 1 // return "[object Object]1"

Where as second one is just block statement not object

{} + 1 // return 1

answered Jul 28 '19 at 11:37

Code Maniac

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score 3 · Answer 2 · answered Jul 28 '19 at 11:37

When not in an expression context, {} will be interpreted as an empty block, so

{} + 1

is like

{
  // no statements here
}
+1

where the + is the unary plus operator. The {} here does not denote an object literal, but a block, and the +1 is the final (and only) expression evaluated, so that's what the console will display if you're just typing into the console.

If you are in an expression context, {} + 1 will indeed evaluate to [object Object]1:

console.log(
  {} + 1
);

Implicit type coercion with objects

2 Answers2