Once upon a time, there are creature that destructive, seeking a clumpsy developer to lose their guard and attack them with their claw.
The var
is very wild because var
has no block scope which mean they are "the beast" that visible outside blocks. here some example:
if(true){
var a = 'var is wild'
}
alert(a);
since var
ignore blocks, var a
will become global variable and browser will show alert that contain variable a
value.
var
also can't be block-or
and loop-local
which mean it's just ignore for loop
block.
if you're using var
inside function, then var
become a function-level variable
function sayHi() {
if (true) {
var a = "Hi";
}
alert(a); // works
}
sayHi();
alert(a); // ReferenceError: a is not defined
when we create var
and redeclare it below the first variable, they will ignore the old one and use newest variable value
var user = "Pete";
var user = "John"; // this "var" does nothing (already declared)
// ...it doesn't trigger an error
alert(user); // John
remember when I mention that var
is a global variable, it's also mean that they are able to declare below their use, so technically move them above
function sayHi() {
phrase = "Hello";
alert(phrase);
var phrase;
}
sayHi();
as same as with
function sayHi() {
var phrase;
phrase = "Hello";
alert(phrase);
}
sayHi();
because of this behaviour, I suggest you to use let
and cost
instead of var
they are more modern and clean in term of block.
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