🔸eval

JS ⟩ concepts ⟩ scope ⟩ global ⟩ global object ⟩ eval()

evaluates script (in the current scope) and returns its completion value.

🛑 Never use eval()❗

⭐️

eval has its own scope in strict mode.❗

• eval is a special identifier.

Functions created with the Function constructor always are created in the global scope. 👉 MDN

🗺️

👥

• in strict mode, script in eval has its own scope.

💈範例

• 💈範例：

  • property attribute

• replit： eval

// 'use strict';                // ⭐ toggle sloppy/strict mode
const { log } = console;
const { tryEval } = require('./helpers/ErrorHandling.js');

// ⭐ eval code in a function
function f() {
    
    let x = 1;

    // ⭐ access local variables
    log(eval('x'));                        // 1
    eval('x = 2');
    log(eval('x'));                        // 2
    
    // ⭐ call eval in another function
    let r1 = tryEval(`x = 3`);
    log(r1);                               // { value: 3 }
    log(eval('x'));                        // 2 (`x` not affected❗)

    // ⭐ declare new variables
    eval(`var y = 3`);        // ⭐ in strict mode, var `y` is local to eval❗
    log(`y = ${y}`);          // 'y = 3' (in sloppy mode, it's OK)
    
    eval(`let z = 500`);      // ⭐ "let/const" is ALWALS local to eval❗
    // log(`z = ${z}`);       // ⛔ ReferenceError
    // ReferenceError: z is not defined

    // ⭐ declare function (can access variables in the current scope)
    eval(`function g() { return x += 10 }`);
    log(`g() = ${g()}`);     // 12
    log(x);                  // 12
}

// execute it
f();

// log
[
    tryEval(`300`),    // { value: 300 }
    tryEval(`+40n`),   // { error: TypeError }
    
].forEach(x => log(x));

📁 ErrorHandling.js

// ⭐ try to eval an `exprStr` (don't include any variables, literals only!)
// returns: result (object)
//   • access `result.value` (any) if successful,
//   • access `result.error` (Error object) otherwise.
function tryEval(exprStr) {

    let result = {};

    try { result.value = eval(exprStr); }
    catch (err) { result.error = err; }

    return result;
}


// ⭐ try to transform a value
// returns:
//   • transformed result (object)
//     (access result.value if successful, result.error otherwise)
function tryTransform(value, transform) {

    let result = {};

    try { result.value = transform(value); }
    catch (err) { result.error = err; }

    return result;
}

// ⭐ get the result of transforming a value
// returns:
//   • transformed value (any)
//   • error type name (string)
function getResult(x, transform) {
    let result = tryTransform(x, transform);
    return result.error ? result.error.name : result.value;
}

// export
module.exports = { tryTransform, getResult, tryEval };

📗

• JavaScript: The Definitive Guide ⟩ 4.12 Evaluation Expressions (eval)

• Eloquent JS ⟩ Evaluating Data as Code

📘

• eval()
• Function ⟩ Difference between Function constructor and function declaration