add backspace function

main.js

@@ -178,6 +178,32 @@ function keyPress(c) {
 	currentInput.setSelectionRange(selectionStart+1, selectionStart+1);
 }
 
+function backspace() {
+	currentInput.focus();
+	if (selectionEnd === 0) {
+		return;
+	}
+
+	let begin = null;
+	if (selectionStart === selectionEnd) {
+		begin = currentInput.value.slice(0, selectionStart-1);
+	} else {
+		begin = currentInput.value.slice(0, selectionStart);
+	}
+
+	const end = currentInput.value.slice(selectionEnd);
+
+	currentInput.value = begin + end;
+
+	if (selectionStart === 0) {
+		currentInput.setSelectionRange(0, 0);
+	} else if (selectionStart === selectionEnd) {
+		currentInput.setSelectionRange(selectionStart-1, selectionStart-1);
+	} else {
+		currentInput.setSelectionRange(selectionStart, selectionStart);
+	}
+}
+
 document.querySelector("#expr").addEventListener("focus", setCurrentInput);
 document.querySelector("#modulus").addEventListener("focus", setCurrentInput);
 
@@ -205,5 +231,6 @@ document.querySelector("#pow").addEventListener("click", () => keyPress("^"));
 document.querySelector("#clear").addEventListener("click", clear);
 document.querySelector("#enter").addEventListener("click", calculate);
 
-// TODO backspace might be tricky
+document.querySelector("#backspace").addEventListener("click", backspace);
+
 // TODO implement square root and inverse keys