Disallow non-string content.

Without this it’s quite easy to accidentally pass an array, which may
seem to just work but will have the wrong length, causing further
operations at or after the affected location to be wrong.

src/MagicString/index.js

@@ -19,6 +19,10 @@ MagicString.prototype = {
 	},
 
 	append: function ( content ) {
+		if ( typeof content !== 'string' ) {
+			throw new TypeError( 'appended content must be a string' );
+		}
+
 		this.str += content;
 		return this;
 	},
@@ -165,6 +169,10 @@ MagicString.prototype = {
 	},
 
 	insert: function ( index, content ) {
+		if ( typeof content !== 'string' ) {
+			throw new TypeError( 'inserted content must be a string' );
+		}
+
 		if ( index === 0 ) {
 			this.prepend( content );
 		} else if ( index === this.original.length ) {
@@ -224,6 +232,10 @@ MagicString.prototype = {
 	},
 
 	replace: function ( start, end, content ) {
+		if ( typeof content !== 'string' ) {
+			throw new TypeError( 'replacement content must be a string' );
+		}
+
 		var firstChar, lastChar, d;
 
 		firstChar = this.locate( start );

test/index.js

@@ -30,6 +30,14 @@ describe( 'MagicString', function () {
 			var s = new MagicString( 'abcdefghijkl' );
 			assert.strictEqual( s.append( 'xyz' ), s );
 		});
+
+		it( 'should throw when given non-string content', function () {
+			var s = new MagicString( '' );
+			assert.throws(
+				function () { s.append( [] ); },
+				TypeError
+			);
+		});
 	});
 
 	describe( 'clone', function () {
@@ -263,6 +271,14 @@ describe( 'MagicString', function () {
 			assert.equal( s.insert(1, '1').toString(), 'a1b' );
 			assert.equal( s.insert(1, '2').toString(), 'a12b' );
 		});
+
+		it( 'should throw when given non-string content', function () {
+			var s = new MagicString( '' );
+			assert.throws(
+				function () { s.insert( 0, [] ); },
+				TypeError
+			);
+		});
 	});
 
 	describe( 'locate', function () {
@@ -466,6 +482,14 @@ describe( 'MagicString', function () {
 			var s = new MagicString( 'abcdefghijkl' );
 			assert.strictEqual( s.replace( 3, 4, 'D' ), s );
 		});
+
+		it( 'should throw when given non-string content', function () {
+			var s = new MagicString( '' );
+			assert.throws(
+				function () { s.replace( 0, 1, [] ); },
+				TypeError
+			);
+		});
 	});
 
 	describe( 'slice', function () {