行単位でdiffが見たい人生だった… https://github.com/kpdecker/jsdiff

sample.js

require('colors')
var jsdiff = require('diff');
var fs = require("fs");

// var one = 'beep boop';
// var other = 'beep boob blah';

var one = fs.readFileSync("./sampleA.js", {encoding: "utf-8"});
var other = fs.readFileSync("./sampleB.js", {encoding: "utf-8"});

var LineDiff = new jsdiff.Diff(true);
LineDiff.tokenize = function (value) {
    return value.split(/^/m);
};
LineDiff.equals = function (left, right) {
    return left.trim() === right.trim();
};

var diff = LineDiff.diff(one, other);
// var diff = jsdiff.createPatch("test.js", one, other, "a", "b");

printDiff(diff);

function printDiff(diff) {
    function addLineHeader(decorator, str) {
        return str.split("\n").map(function (line, index, array) {
            if (index === array.length - 1 && line === "") {
                return line;
            } else {
                return decorator + line;
            }
        }).join("\n");
    }

    diff.forEach(function (part) {
        // green for additions, red for deletions
        // grey for common parts
        if (part.added) {
            process.stderr.write(addLineHeader("+", part.value).green);
        } else if (part.removed) {
            process.stderr.write(addLineHeader("-", part.value).red);
        } else {
            process.stderr.write(addLineHeader(" ", part.value).grey);
        }
    });

}