What difference does it makes to parse something as an object literal rather than as a block?

Question

Sorry for my ignorance on JavaScript basic concepts.

It boils down to this:

Literal - A value found directly in the script. Examples:

Answer 1

First, don't eval JSON, use JSON.parse on the String source

---

A block is a "group of expressions" for example,

let x = 0;
if (true) {
    // this is a block
    ++x;
}

However, equally this is also a block

let x = 0;
{ // hi there, I'm a block!
    ++x;
}

This means when the interpreter sees block notation, it assumes a block even if you do something like this

{ // this is treated as a block
    foo: ++x
}

Here, foo acts as a label rather than a property name and if you try to do more complex things with the attempted object literal, you're going to get a Syntax Error.

When you want to write an Object literal ambiguously like this, the solution is to force the interpreter into "expression mode" explicitly by providing parenthesis

({ // this is definately an Object literal
    foo: ++x
})

Answer 2

A group that begins with a { and ends with a } is treated as either object literal or a block depending on context*.

Within an expression context the group is interpreted as an object literal. Writing a block within an expression context will generate a syntax error:

// Valid code:
foo = {a:b};
({a:b});

// Syntax errors:
foo = {var a = b};
({var a = b});

Outside of an expression context the group is interpreted as a block. Depending on exactly how the code is written, an object literal written outside of an expression context is either a syntax error or will be interpreted as a label.

*note: In the ECMAscript spec the word "context" is used to mean something specific. My use of the word here is with the general meaning in computer science with regards to parsing.