Bookmarklet using window.prompt as a REPL interface for current page. Keywordable.

eval-repl-bookmarklet.js

javascript: (() => {
	let doPrompt = true;
	let promtLabel = '';
	let promptValue = '%s';
	if (promptValue == '\u0025s') {
		promtLabel = 'Eval:';
		promptValue = '/* Expression like */ 1 + 1';
	} else {
		doPrompt = false;
	}
	do {
		try {
			if (doPrompt) {
				promptValue = prompt(promtLabel, promptValue);
				if (promptValue === null) {
					break;
				}
			} else {
				doPrompt = true;
			}
			let result = eval(promptValue);
			promtLabel = promptValue + '\n=\n' + result;
			promptValue = promptValue/*.replace(/ \/\/= .*?$/, '') + ' //= ' + result;*/;
		} catch (e) {
			promtLabel = e;
		}
	} while (true);
})()

• For "I don't feel like launching devtools just to execute simple onleliner, like document.forms[0].method = 'GET'" or such.
• OK (enter) evaluates input (prompt).
• Next prompt displays previous input and result (or error) and prefills the result into input.
• Cancel (escape) exits.
• (!) won't work for pages with strict CSP (like GitHub), as any other bookmarklet (https://bugzilla.mozilla.org/show_bug.cgi?id=866522).
• "Keywordable" means if you set a keyword like ! then you can do ! 1 + 1 (enter) in URL bar and get the result. Again, only if page context permits execution and eval. So you can do ! document.forms[0].method = 'get' (enter)(escape), or continue further.