setOperand function for OpenCalc

setOperand.js

setOperand(op: Operation): ?Operation {
		let newNumber: ?string;
		if (!Validator.validDigit(op.stringVal, this.queue)) {
			return null;
		}

		let lastOp: ?Operation = lastOrNull(this.queue);

		if (lastOp && (
				this.cleared ||
				lastOp.operationArgs.has(OperationArgs.NotParseable) ||
				op.operationArgs.has(OperationArgs.NotParseable) && lastOp.operationType === OperationType.Constant)
		) {
			this.queue.pop();
		} else if (lastOp && lastOp.operationType === OperationType.Constant) {
			const lastNumber = this.queue.pop();
			newNumber = lastNumber.stringVal + op.stringVal;
		}

		if (newNumber) {
			return newOperation(newNumber, this.queue);
		} else {
			return op;
		}
	}