Msg-switch NPM

msg-switch

An easy way to display error messages for a predefined set of error codes.

Motivation

During our transition from AngularJS (v1.6.x) to VueJS, we encountered a problem. We struggled to find an easy way to migrate an AngularJS component relying on ngMessages directive.

The requirements are as follows:

Define lists of messages.
Re-use these lists.
Override message in these lists on a per-case basis.
Extend these lists.

Examples

Basic Example

In this basic example the components are used to display an error message based on an error code. Basically, it behaves as a huge switch statement: chooses the right message to be displayed. But unlike a switch statement it should be able

// StdError.vue
<MsgSwitch :value="code">
  <!--
    User-defined custom messages comes first,
    such that they overwrite the default messages.
  -->
  <slot :value="value" />

  <!-- Here the standard error messages come -->
  <MsgCase when="unautheticated">Please log in!</MsgCase>
  <MsgCase when="not_found">Not found.</MsgCase>
  <MsgCase when="forbidden">You do not enough permissions.</MsgCase>
  <!-- A standrard wildcard case. -->
  <MsgCase when="*">
    <template slot-scope="{ value }">
      Unknown error: <pre>{{value}}</pre>.
    </template>
  </MsgCase>
</MsgSwitch>

Than this component is used as follows:

// App.vue
<StdError :value="error.code">
  <MsgCase when="not_found">The document was not found.</MsgCase>
  <MsgCase when="forbidden">You are not allowed to edit the document.</MsgCase>
  <MsgCase when="dynamic">Message for error code missing in StdError.</MsgCase>
</StdError>

In the example above the following scenarios are possible:

If the error code is unautheticated, the standard error message listed in StdError would be displayed: Please log in!.
If the error code is not_found, the custom error message would be displayed: The document was not found..
If the error code is dynamic, the custom error message would be displayed: Message for error code missing in StdError..

Access Parent Scope Example

In this example an additional context is passed to MsgSwitch. This context is treated as an opaque value and is passed as is to MsgCase components. It can be used inside MsgCase components using scoped slots as show below.

// StdErrorParentScope.vue
<MsgSwitch :value="error.code">
  <!--
    User-defined custom messages comes first,
    such that they overwrite the default messages.
  -->
  <slot :value="error.code" />

  <!-- Here the standard error messages come -->
  <MsgCase when="unautheticated">Please log in!</MsgCase>
  <MsgCase when="not_found" #default>
    Resource {{ error.id }} was not found.
  </MsgCase>
  <MsgCase when="forbidden" #default>
    You do not enough permissions to {{ error.method }} {{ error.resourceName }}.
  </MsgCase>
  <!-- A standrard wildcard case. -->
  <MsgCase when="*" #default="{ value }">
    Unknown error (<code>{{value}}</code>): <pre>{{ error }}</pre>.
  </MsgCase>
</MsgSwitch>

Than this component is used as follows:

// App.vue
<StdErrorParentScope :error="error">
  <MsgCase when="not_found" #default>
    User {{ error.id }} was not found.
  </MsgCase>
  <MsgCase when="dynamic" #default>
    Message for error code missing in StdError:
    <pre>{{ error.someDebugInfo }}</pre>.
  </MsgCase>
</StdErrorWithContext>

Context Example

Update 2019-02-08: Vue 2.6 added support for the new v-slot syntax. This made the ctx prop redundant. Checkout Access Parent Scope Example. For more details about v-slot please refer to Vue 2.6 release blog post and updated Vue slot docs.

// StdErrorWithContext.vue
<MsgSwitch :value="code" :ctx="someObject">
  <!--
    User-defined custom messages comes first,
    such that they overwrite the default messages.
  -->
  <slot :value="value" />

  <!-- Here the standard error messages come -->
  <MsgCase when="unautheticated">Please log in!</MsgCase>
  <MsgCase when="not_found">
    <template slot-scope="{ ctx }">
      Resource {{ ctx.id }} was not found.
    </template>
  </MsgCase>
  <MsgCase when="forbidden">
    <template slot-scope="{ ctx }">
      You do not enough permissions to {{ ctx.method }} {{ ctx.resourceName }}.
    </template>
  </MsgCase>
  <!-- A standrard wildcard case. -->
  <MsgCase when="*">
    <template slot-scope="{ ctx, value }">
      Unknown error (<code>{{value}}</code>): <pre>{{ ctx }}</pre>.
    </template>
  </MsgCase>
</MsgSwitch>

Than this component is used as follows:

// App.vue
<StdErrorWithContext :value="error.code">
  <MsgCase when="not_found">
    <template slot-scope="{ ctx }">
      User {{ ctx.id }} was not found.
    </template>
  </MsgCase>
  <MsgCase when="dynamic">
    <template slot-scope="{ ctx }">
      Message for error code missing in StdError:
      <pre>{{ ctx.someDebugInfo }}</pre>.
    </template>
  </MsgCase>
</StdErrorWithContext>