Today, I learned to parse json strings and used an eval() method. Why do I need to add brackets when parsing strings? Can't touch it. It turns out that the {} statement block in javascript has ambiguity, and it will make an error without brackets. Understanding this ambiguity is of great help to our understanding of javascript code.
1. Two meanings of {} statement block
Represents a statement block
a. You can use {} to enclose the code in javascript to facilitate the management of code in the editor. Because javascript does not have a block-level scope, this writing is harmless.
{
//some code...
}
b. In javascript, conditional judgment statements, loop statements, and functions all require {} statement blocks to integrate the code
Object literal
var box = {
name:'kuoaho',
age:21
}
//At this time, [code] can be assigned to a variable as an expression
//In fact, the object literal is an expression that can generate object values
2. What happens if the object literal is not used as an assignment expression?
example:
{name:'kuoao'} //No error was reported, but no object was created either {name:'kuohao',age} //Report an error
From the above, we can see that the object literal can only be assigned as an expression. The first way is written is not wrong, but JavaScript parses it as a label statement.
analysis:
{name:'kuoao'}
//{} A statement block // name:'kuohao', a label statement used to mark the for loop
3. But the problem arises again...
{
name:'kuohao',
age:21
}
//Why does this cause the error? Isn't this how to write the literal object?
Because of the ambiguity of {} in javascript, {} is not only considered an object literal but also a code block.
analysis:
{
name:'kuohao',
age:21
}
A code block, two label statements, if there is no comma, there is no problem at all, so the key is the comma. The separation of the two statements should use a semicolon, so javascript will determine that this is a syntax error
4. Correct writing
({
name:'kuohao',
age:21
})
//Correct writing
() Will convert a statement into an expression, called a statement expression. Isn’t an object literal expression? Why do we still need () to convert?
After adding brackets, this ambiguity can be eliminated, because the code in brackets will be converted into expression evaluation and returned, so the statement block becomes the object literal. It can also be concluded that the object literal must exist as an expression.