Eval () He is an angel and a devil

Eval literal meaning: Evaluate evaluation;

The eval function evaluates a given string containing JavaScript code and tries to execute expressions contained in the string or a series of legal JavaScript statements. The eval function uses the value or reference contained in the last expression or statement as the return value.

Example code:

VaR evalarray = ["ID", "time", "info"]; var E = eval (evalarray); console. log (E); // output: ['id', 'time', 'info']

VaR evalarray = {"ID": 1, "Time": "15:21", "info": "student"}; var E = eval (evalarray); console. log (E); // output {ID: 1, time: '15: 21', Info: 'student '}

VaR x = 2, y = 5; var E = eval ("X + Y"); console. Log (E); // output 7

Console. Log (eval ('{Timer: "Hello"}'); // output hello

The purpose of parentheses is to force the eval function to forcibly convert the expressions in parentheses into objects rather than execute them as statements when evaluating JavaScript code.

For example, object literal quantity {},

Console. Log (eval ("{}"); // output undefinedconsole. Log (eval ("({})"); // output {}

Without the outer brackets, Eval identifies the braces as the start and end marks of the JavaScript code block,

Then {} is considered to have executed an empty statement. Therefore, the following two execution results are different.

Evel accepts arbitrary strings and processes them as JavaScript code. When the problematic code is known in advance (not determined at the runtime), there is no reason to use eval (). To be precise, it is the devil!

Eval () He is an angel and a devil