#25 Optionals with default values aren't being hydrated with the default (#26)

* #25 Optionals with default values aren't being hydrated with the default

* Fix lint warning

Author: DrewImm

src/argument-parser.ts

@@ -64,6 +64,15 @@ export class ArgumentParser {
 			}
 		}
 
+		for (const option of this.command.options) {
+			if (
+				matches[option.key] === undefined &&
+				option.default !== undefined
+			) {
+				matches[option.key] = option.default;
+			}
+		}
+
 		if (this.positionalIndex < requiredPositionals.length) {
 			throw new ArgumentError(
 				`Missing positional arguments: ${requiredPositionals
@@ -185,7 +194,8 @@ export class ArgumentParser {
 			const choices = option.choices.join(', ');
 
 			throw new ArgumentError(
-				`Invalid choice for argument: ${option.key} (${value}). Allowed choices are [${choices}]`
+				`Invalid choice for argument: ${option.key} (${value}). ` +
+					`Allowed choices are [${choices}]`
 			);
 		}
 	}

test/spec/argument-parser.spec.ts

@@ -105,6 +105,40 @@ describe('ArgumentParser', () => {
 		});
 	});
 
+	it('populates default values for positionals', () => {
+		command.positional = [
+			{
+				key: 'input',
+				type: OptionType.string,
+				default: 'defaultInput.txt',
+			},
+		];
+		command.options = [];
+		command.init();
+
+		const result = parser.parse([]);
+		expect(result).toEqual({
+			input: 'defaultInput.txt',
+		});
+	});
+
+	it('populates default values for optionals', () => {
+		command.positional = [];
+		command.options = [
+			{
+				key: 'defaultOption',
+				type: OptionType.string,
+				default: 'defaultValue',
+			},
+		];
+		command.init();
+
+		const result = parser.parse([]);
+		expect(result).toEqual({
+			defaultOption: 'defaultValue',
+		});
+	});
+
 	it('throws an error for missing value for string option', () => {
 		const args = ['--output'];
 		expect(() => parser.parse(args)).toThrowError(