Why Node REPL evaluates expressions differently depending on whether it's assigned to variable or not?

Question

Let's consider the following input in terminal

✗ node
Welcome to Node.js v13.1.0.
Type ".help" for more information.

> let a = 13
undefined
> {} + a.toString()
13

> // but
undefined

> let b = {} + a.toString()
undefined
> b
'[object Object]13'

The question is why when you evaluate {} + a.toString() REPL will show a digit 13, but when you assign it to a variable it equals to the expected string '[object Object]13'?

This behaviour happens at least in the V8 (Node and Chrome).

[related](https://stackoverflow.com/questions/53306680/why-does-null-give-a-syntaxerror/53307328?r=SearchResults#53307328) — Jonas Wilms, Nov 08 '19 at 18:18
repls/consoles are weird. For example, `{a+3}` is a block, `{a:3}` isn't. — georg, Nov 08 '19 at 18:45

score 1 · Accepted Answer · answered Nov 08 '19 at 18:22

1

The root problem here is that { ... } in JS is syntactically either a block statement or an object literal expression, depending on the context (wether a statement or an expression is expected). In your second case, it is clearly an expression:

 let b = /*expression context*/

thus {} is an object literal there. In the first case, it is in a statement context, and thus is interpreted as:

  {} // an empty block statement
  + a.toString() // a unary plus operator on "13"

answered Nov 08 '19 at 18:22

Jonas Wilms

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Which 100% makes sense. Thanks! – Vlad Miller Nov 08 '19 at 18:23
1

Glad to help :) – Jonas Wilms Nov 08 '19 at 18:25

Why Node REPL evaluates expressions differently depending on whether it's assigned to variable or not?

1 Answers1