tessl/pypi-pyexecjs

Run JavaScript code from Python with automatic runtime selection

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JavaScript Execution

Name: tessl/pypi-pyexecjs
Author: tessl

Core functionality for evaluating and executing JavaScript code from Python applications.

Overview

PyExecJS provides three main execution modes: evaluation (returning results), execution (returning output), and compilation (creating reusable contexts). All functions automatically select the best available JavaScript runtime unless specifically configured.

Core Functions

eval()

Evaluate JavaScript expressions and return the computed result.

def eval(source, cwd=None):
    """
    Evaluate JavaScript source code and return the result.
    
    Args:
        source (str): JavaScript source code to evaluate
        cwd (str, optional): Working directory for execution
        
    Returns:
        any: Result of JavaScript evaluation (int, float, str, bool, list, dict, or None)
        
    Raises:
        ProgramError: When JavaScript code contains syntax or runtime errors
        RuntimeError: When JavaScript runtime engine encounters errors
        RuntimeUnavailableError: When no JavaScript runtime is available
    """

exec_()

Execute JavaScript code and return the standard output.

def exec_(source, cwd=None):
    """
    Execute JavaScript source code and return stdout output.
    
    Args:
        source (str): JavaScript source code to execute
        cwd (str, optional): Working directory for execution
        
    Returns:
        str: Standard output from JavaScript execution
        
    Raises:
        ProgramError: When JavaScript code contains syntax or runtime errors
        RuntimeError: When JavaScript runtime engine encounters errors
        RuntimeUnavailableError: When no JavaScript runtime is available
    """

compile()

Compile JavaScript source code into a reusable execution context.

def compile(source, cwd=None):
    """
    Compile JavaScript source into a reusable context object.
    
    Args:
        source (str): JavaScript source code to compile
        cwd (str, optional): Working directory for compilation
        
    Returns:
        Context object: Compiled JavaScript execution context with eval(), exec_(), and call() methods
        
    Raises:
        ProgramError: When JavaScript code contains syntax errors
        RuntimeError: When JavaScript runtime engine encounters errors
        RuntimeUnavailableError: When no JavaScript runtime is available
    """

Usage Examples

Basic Evaluation

import execjs

# Simple expressions
result = execjs.eval("2 + 3 * 4")  # 14
name = execjs.eval("'Hello'.toUpperCase()")  # "HELLO"

# Complex objects
data = execjs.eval("({name: 'John', age: 30, scores: [85, 92, 78]})")
# Returns: {'name': 'John', 'age': 30, 'scores': [85, 92, 78]}

Execution with Output

import execjs

# Console output
output = execjs.exec_("console.log('Processing...'); console.log('Done!')")
# Returns: "Processing...\nDone!\n"

# Multiple statements
code = """
for (var i = 1; i <= 3; i++) {
    console.log('Step ' + i);
}
"""
output = execjs.exec_(code)
# Returns: "Step 1\nStep 2\nStep 3\n"

Compilation and Reuse

import execjs

# Compile JavaScript with functions
js_code = """
var math = {
    add: function(a, b) { return a + b; },
    multiply: function(a, b) { return a * b; },
    factorial: function(n) {
        if (n <= 1) return 1;
        return n * this.factorial(n - 1);
    }
};
"""

ctx = execjs.compile(js_code)

# Use compiled context multiple times
result1 = ctx.call("math.add", 5, 3)       # 8
result2 = ctx.call("math.multiply", 4, 7)  # 28  
result3 = ctx.call("math.factorial", 5)    # 120

Working Directory Support

All execution functions support specifying a working directory for JavaScript execution:

import execjs
import os

# Execute in specific directory
result = execjs.eval("typeof require !== 'undefined'", cwd="/path/to/node/project")

# Useful for Node.js modules that depend on relative paths
ctx = execjs.compile("var fs = require('fs');", cwd="/home/user/project")

Type Conversion

PyExecJS automatically converts between JavaScript and Python types:

JavaScript Type	Python Type
`number`	`int` or `float`
`string`	`str`
`boolean`	`bool`
`null`	`None`
`undefined`	`None`
`Array`	`list`
`Object`	`dict`

Error Handling

import execjs

try:
    result = execjs.eval("invalidSyntax(")
except execjs.ProgramError as e:
    print(f"JavaScript error: {e}")

try:
    result = execjs.eval("throw new Error('Custom error')")  
except execjs.ProgramError as e:
    print(f"JavaScript threw: {e}")

try:
    # This will fail if no runtime is available
    result = execjs.eval("2 + 2")
except execjs.RuntimeUnavailableError as e:
    print(f"No JavaScript runtime available: {e}")

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