Adding ISBLANK spreadsheet function

js/spreadsheet.js

 			case 'FLOOR': return this.#funcFloor(args, address);
 			case 'IF': return this.#funcIf(args, address);
 			case 'IFS': return this.#funcIfs(args, address);
+			case 'ISBLANK': return this.#funcIsBlank(args, address);
 			case 'LN': return this.#funcLn(args, address);
 			case 'LOG': return this.#funcLog(args, address);
 			case 'LOWER': return this.#funcLower(args, address);
 	}
 
 	/**
+	 * `IFBLANK(value)` - Returns `TRUE` if the given value is blank, otherwise
+	 * `FALSE`.
+	 *
+	 * @param {Array} args - raw arguments
+	 * @param {CellAddress} address - expression location
+	 * @returns {CellValue} - result
+	 */
+	#funcIsBlank(args, address) {
+		if (args.length !== 1) {
+			throw new CellEvaluationException("IFBLANK expects 1 argument");
+		}
+		const arg = this.#evaluate(args[0], address);
+		if (!(arg instanceof CellValue)) {
+			throw new CellEvaluationException("IFBLANK expcts 1 argument");
+		}
+		return CellValue.fromValue(arg.type === CellValue.TYPE_BLANK);
+	}
+
+	/**
 	 * `LN(value)` - Natural log.
 	 *
 	 * @param {Array} args - raw arguments

php/spreadsheet.php

 			case 'FLOOR': return $this->funcFloor($args, $address);
 			case 'IF': return $this->funcIf($args, $address);
 			case 'IFS': return $this->funcIfs($args, $address);
+			case 'ISBLANK': return $this->funcIsBlank($args, $address);
 			case 'LN': return $this->funcLn($args, $address);
 			case 'LOG': return $this->funcLog($args, $address);
 			case 'LOWER': return $this->funcLower($args, $address);
 	}
 
 	/**
+	 * `IFBLANK(value)` - Returns `TRUE` if the given value is blank, otherwise
+	 * `FALSE`.
+	 *
+	 * @param array $args  raw arguments
+	 * @param CellAddress $address  expression location
+	 * @return CellValue  result
+	 */
+	private function funcIsBlank(array $args, CellAddress $address): CellValue {
+		if (sizeof($args) !== 1) {
+			throw new CellEvaluationException("IFBLANK expects 1 argument");
+		}
+		$arg = $this->evaluate($args[0], $address);
+		if (!($arg instanceof CellValue)) {
+			throw new CellEvaluationException("IFBLANK expcts 1 argument");
+		}
+		return CellValue::fromValue($arg->type === CellValue::TYPE_BLANK);
+	}
+
+	/**
 	 * `LN(value)` - Natural log.
 	 *
 	 * @param array $args  raw arguments

spreadsheet.md

 | `FLOOR(`_x_`)` | Rounds a value down |
 | `IF(`_test_`,` _trueval_`,` _falseval_`)` | Returns _trueval_ if _test_ evaluates to `TRUE`, otherwise returns _falseval_.
 | `IFS(`_test1_`,` _val1_`,` _test2_`,` _val2_`,` ... `,` _fallbackval_`)` | Performs multiple if tests. If _test1_ is `TRUE`, returns _val1_. If _test2_ is `TRUE`, returns _val2_. Etc. If no tests are `TRUE`, the final _fallbackval_ is returned. Takes an odd number of arguments of 3 or more. |
+| `ISBLANK(`_x_`)` | True if the value or cell is blank |
 | `LN(`_x_`)` | Natural logarithm |
 | `LOG(`_x_`,[` _base_`])` | Logarithm with a given _base_, or 10 if _base_ omitted |
 | `LOWER(`_x_`)` | Lowercase of a text value |