More friendly messages for non-string

index.js

@@ -6,6 +6,12 @@ function parseJson (txt, reviver, context) {
   try {
     return JSON.parse(txt, reviver)
   } catch (e) {
+    if (typeof txt !== 'string') {
+      const isEmptyArray = Array.isArray(txt) && txt.length === 0
+      const errorMessage = 'Cannot parse ' +
+      (isEmptyArray ? 'an empty array' : String(txt))
+      throw new TypeError(errorMessage)
+    }
     const syntaxErr = e.message.match(/^Unexpected token.*position\s+(\d+)/i)
     const errIdx = syntaxErr
     ? +syntaxErr[1]

test/index.js

@@ -14,3 +14,70 @@ test('parses JSON', t => {
   t.deepEqual(JSON.parse(data), parseJson(data), 'does the same thing')
   t.done()
 })
+
+test('throws SyntaxError for unexpected token', t => {
+  const data = 'foo'
+  t.throws(
+    () => parseJson(data),
+    new SyntaxError('Unexpected token o in JSON at position 1 while parsing near \'foo\'')
+  )
+  t.done()
+})
+
+test('throws SyntaxError for unexpected end of JSON', t => {
+  const data = '{"foo: bar}'
+  t.throws(
+    () => parseJson(data),
+    new SyntaxError('Unexpected end of JSON input while parsing near \'{"foo: bar}\'')
+  )
+  t.done()
+})
+
+test('throws SyntaxError for unexpected number', t => {
+  const data = '[[1,2],{3,3,3,3,3}]'
+  t.throws(
+    () => parseJson(data),
+    new SyntaxError('Unexpected number in JSON at position 8')
+  )
+  t.done()
+})
+
+test('SyntaxError with less context (limited start)', t => {
+  const data = '{"6543210'
+  t.throws(
+    () => parseJson(data, null, 3),
+    new SyntaxError('Unexpected end of JSON input while parsing near \'...3210\''))
+  t.done()
+})
+
+test('SyntaxError with less context (limited end)', t => {
+  const data = 'abcde'
+  t.throws(
+    () => parseJson(data, null, 2),
+    new SyntaxError('Unexpected token a in JSON at position 0 while parsing near \'ab...\''))
+  t.done()
+})
+
+test('throws TypeError for undefined', t => {
+  t.throws(
+    () => parseJson(undefined),
+    new TypeError('Cannot parse undefined')
+  )
+  t.done()
+})
+
+test('throws TypeError for non-strings', t => {
+  t.throws(
+    () => parseJson(new Map()),
+    new TypeError('Cannot parse [object Map]')
+  )
+  t.done()
+})
+
+test('throws TypeError for empty arrays', t => {
+  t.throws(
+    () => parseJson([]),
+    new TypeError('Cannot parse an empty array')
+  )
+  t.done()
+})