JavaScript eval tricks
By default, eval
evaluates code in the current context.
var x = 0;
function foo() {
var x = 5;
eval("x = 100");
return x;
}
x; // 0
foo(); // 100
x; // 0
However, there’s a trick to confuse everyone.
If we assign the eval
function to a variable, the function inside that
variable “detaches” from the current context: this
becomes the global object.
var x = 0;
function bar() {
var x = 5,
evil = eval;
evil("x = 100");
return x;
}
x; // 0
bar(); // 5
x; // 100
This is how you could write the function (don’t do that!) to make everyone superconfused.
var x = 0;
function baz() {
var x = 5,
evil = eval.call(this, "eval");
evil("x = 100");
return x;
}
x; // 0
baz(); // 100
x; // 0
JS is cool.
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