I generate AST from {} + {} as follows
{
"type": "Program",
"start": 0,
"end": 5,
"body": [
{
"type": "BlockStatement",
"start": 0,
"end": 2,
"body": []
},
{
"type": "ExpressionStatement",
"start": 2,
"end": 5,
"expression": {
"type": "UnaryExpression",
"start": 2,
"end": 5,
"operator": "+",
"prefix": true,
"argument": {
"type": "ObjectExpression",
"start": 3,
"end": 5,
"properties": []
}
}
}
],
"sourceType": "module"
}
from AST , the first {} is still converted into a code block, and + is a unary operator, but geese are indeed treated as arithmetic operators "[object Object] [object Object]" in the expression of chrome , which may be a feature of chrome (nonsense, I don't know why), in firefox
the following is you don't know the JavaScript (medium volume) Chapter 5 P102 original text:
< hr >
there is another pit that is often mentioned (involving forced type conversion, see Chapter 4):
[] + {}; // "[object Object]"
{} + []; // 0
on the surface, the + operator produces different results depending on the first Operand ([] or {}), but it is not. In the first line of code, {} appears in the + operator expression, so it is treated as a value (empty object) . Chapter
4 mentioned that [] will be cast to "" and {} will be cast to "[object Object]" .
but in the second line of code, {} is treated as a separate empty code block (nothing is done) . There is no need for a semicolon at the end of the code block, so there is no syntax problem here. Finally, + [] explicitly cast (see Chapter 4) to 0.
< hr >
this series covers all the messy problems in js in great detail
because you wrote {} + {} in parentheses
