How do I transpile an *expression* with babel?

Question

Given the following:

require('babel-core').transform('3').code

Is there a way to get this to return 3 (an expression) instead of 3; (a statement)?

I've tried:

• Searching the web and various sites for 'babel expression', 'babel transpile expression', and the like.
• Passing (3), but that also was transformed into 3;.
• Poking the babel internals to figure out what's going on, but I don't know enough about how it works to sort this out.
• Passing the option minified: true, which claims to remove the trailing semicolon but doesn't actually seem to do anything.

Right now I'm using .replace(/;$/, ''), which works but seems absurd and error-prone.

Answer 1

@loganfsmyth provided the missing link on BabelJS.slack -- parserOpts.allowReturnOutsideFunction. Here's the code I came up with:

const babel = require('babel-core');

let script = 'return ((selector, callback) => Array.prototype.map.call(document.querySelectorAll(selector), callback))("#main table a.companylist",a => a.innerText)';

let transform = babel.transform(script, {
    ast: false,
    code: true,
    comments: false,
    compact: true,
    minified: true,
    presets: [
        ['env', {
            targets: {
                browsers: ['> 1%', 'last 2 Firefox versions', 'last 2 Chrome versions', 'last 2 Edge versions', 'last 2 Safari versions', 'Firefox ESR', 'IE >= 8'],
            }
        }]
    ],
    parserOpts: {
        allowReturnOutsideFunction: true,
    }
});

let code = `return function(){${transform.code}}()`;

Output:

return function(){"use strict";return function(selector,callback){return Array.prototype.map.call(document.querySelectorAll(selector),callback)}("#main table a.companylist",function(a){return a.innerText});}()

My input script looks a bit funny just because of how I generated it, but you can see that it transformed those arrow expressions into regular functions and the whole thing is still an expression.

N.B. you may want to modify that last line to something like

let code = `(function(){${transform.code}})()`;

depending on your needs. I needed mine to be returned.