Adding a type guard for AxiosError (#2949)

Co-authored-by: Jason Kwok <JasonHK@users.noreply.github.com>

test/specs/api.spec.js

@@ -41,6 +41,10 @@ describe('static api', function () {
     expect(typeof axios.CancelToken).toEqual('function');
     expect(typeof axios.isCancel).toEqual('function');
   });
+
+  it('should have isAxiosError properties', function () {
+    expect(typeof axios.isAxiosError).toEqual('function');
+  });
 });
 
 describe('instance api', function () {

test/specs/helpers/isAxiosError.spec.js

@@ -0,0 +1,20 @@
+var createError = require('../../../lib/core/createError');
+var enhanceError = require('../../../lib/core/enhanceError');
+var isAxiosError = require('../../../lib/helpers/isAxiosError');
+
+describe('helpers::isAxiosError', function () {
+  it('should return true if the error is created by core::createError', function () {
+    expect(isAxiosError(createError('Boom!', { foo: 'bar' })))
+      .toBe(true);
+  });
+
+  it('should return true if the error is enhanced by core::enhanceError', function () {
+    expect(isAxiosError(enhanceError(new Error('Boom!'), { foo: 'bar' })))
+      .toBe(true);
+  });
+
+  it('should return false if the error is a normal Error instance', function () {
+    expect(isAxiosError(new Error('Boom!')))
+      .toBe(false);
+  });
+});

test/specs/instance.spec.js

@@ -19,6 +19,7 @@ describe('instance', function () {
         'isCancel',
         'all',
         'spread',
+        'isAxiosError',
         'default'].indexOf(prop) > -1) {
         continue;
       }

test/typescript/axios.ts

@@ -358,3 +358,12 @@ axios.get('/user', {
 });
 
 source.cancel('Operation has been canceled.');
+
+// AxiosError
+
+axios.get('/user')
+  .catch((error) => {
+    if (axios.isAxiosError(error)) {
+      const axiosError: AxiosError = error;
+    }
+  });