Client Script Errors — Diagnosis and Resolution

Cross-refs: frappe-syntax-clientscripts (syntax), frappe-impl-clientscripts (workflows), frappe-errors-serverscripts (server-side).

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Error Diagnosis Flowchart

ERROR IN CLIENT SCRIPT
│
├─► TypeError: Cannot read properties of undefined
│   ├─► "frm.doc.fieldname" → Field does not exist on DocType
│   ├─► "r.message.value" → Server returned null/error
│   └─► "row.fieldname" in child table → Row not fetched correctly
│
├─► frappe.call fails silently
│   ├─► Missing error callback → Add error handler
│   ├─► 403 Forbidden → Method not whitelisted (@frappe.whitelist)
│   ├─► 417 Expectation Failed → Server-side frappe.throw()
│   └─► 401 Unauthorized → Session expired or CSRF token invalid
│
├─► Uncaught (in promise) → Missing try/catch on async frappe.call
│
├─► Field appears blank after set_value → Timing issue (setup vs refresh)
│
├─► cur_frm is undefined → Using cur_frm in list/report context
│
└─► frappe.throw() does not prevent save → Used outside validate event

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Error Message → Cause → Fix Table

Error Message	Cause	Fix
TypeError: Cannot read properties of undefined (reading 'fieldname')	Field does not exist on DocType or doc not loaded	ALWAYS check frm.doc exists before accessing fields
TypeError: frm.set_value is not a function	Using cur_frm shortcut that is undefined	ALWAYS use the frm parameter from event handler
Uncaught (in promise)	Unhandled async rejection from frappe.call	ALWAYS wrap async calls in try/catch
CSRFTokenError / 403 with CSRF	Token mismatch after session timeout	ALWAYS use frappe.call() (handles CSRF automatically)
Not permitted / 403 on frappe.call	Server method missing @frappe.whitelist()	ALWAYS add @frappe.whitelist() decorator to API methods
frappe.throw() not preventing save	frappe.throw() used outside validate event	ALWAYS use frappe.throw() only in validate
field not found: xyz in set_query	Fieldname typo or field not in child table	Verify exact fieldname against DocType definition
row.item_code is undefined	Accessing child row wrong — locals not synced	Use frappe.get_doc(cdt, cdn) in child table events
frm.set_value not working	Called in setup before form fully loaded	Move field-setting logic to refresh event
Maximum call stack exceeded	Circular trigger — field change fires own handler	Use frm.flags guard to break recursion

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Critical Error Patterns

1. cur_frm vs frm: The #1 Beginner Mistake

// ❌ WRONG — cur_frm is undefined in many contexts
frappe.ui.form.on('Sales Order', {
    customer(frm) {
        cur_frm.set_value('territory', 'Default');  // BREAKS in list view
    }
});

// ✅ CORRECT — ALWAYS use the frm parameter
frappe.ui.form.on('Sales Order', {
    customer(frm) {
        frm.set_value('territory', 'Default');
    }
});

Rule: NEVER use cur_frm. ALWAYS use the frm parameter passed to every event handler.

2. Async/Await: Silent Failure Without try/catch

// ❌ WRONG — Unhandled rejection crashes silently
frappe.ui.form.on('Sales Order', {
    async customer(frm) {
        let r = await frappe.call({
            method: 'myapp.api.get_data',
            args: { customer: frm.doc.customer }
        });
        frm.set_value('credit_limit', r.message.limit);  // r.message may be null
    }
});

// ✅ CORRECT — try/catch with null check
frappe.ui.form.on('Sales Order', {
    async customer(frm) {
        if (!frm.doc.customer) return;
        try {
            let r = await frappe.call({
                method: 'myapp.api.get_data',
                args: { customer: frm.doc.customer }
            });
            if (r.message) {
                frm.set_value('credit_limit', r.message.limit || 0);
            }
        } catch (error) {
            console.error('Customer fetch failed:', error);
            frappe.show_alert({
                message: __('Could not load customer details'),
                indicator: 'red'
            }, 5);
        }
    }
});

3. Child Table Access: Wrong Pattern

// ❌ WRONG — frm.doc.items[0] may not reflect latest state
frappe.ui.form.on('Sales Order Item', {
    item_code(frm, cdt, cdn) {
        let row = frm.doc.items.find(r => r.name === cdn);  // fragile
        row.rate = 100;  // Does not trigger UI refresh
    }
});

// ✅ CORRECT — Use frappe.get_doc and frappe.model.set_value
frappe.ui.form.on('Sales Order Item', {
    item_code(frm, cdt, cdn) {
        let row = frappe.get_doc(cdt, cdn);
        if (!row.item_code) return;
        frappe.model.set_value(cdt, cdn, 'rate', 100);  // Triggers refresh
    }
});

4. Timing: setup vs refresh

// ❌ WRONG — set_value in setup, form not ready
frappe.ui.form.on('Sales Order', {
    setup(frm) {
        frm.set_value('company', 'My Company');  // May not work
    }
});

// ✅ CORRECT — set_query in setup, set_value in refresh/onload
frappe.ui.form.on('Sales Order', {
    setup(frm) {
        // Filters belong in setup
        frm.set_query('customer', () => ({ filters: { disabled: 0 } }));
    },
    refresh(frm) {
        // Value changes belong in refresh (or onload for new docs)
        if (frm.is_new()) {
            frm.set_value('company', 'My Company');
        }
    }
});

5. frappe.throw() Scope: Only Works in validate

// ❌ WRONG — throw in customer change does NOT prevent save
frappe.ui.form.on('Sales Order', {
    customer(frm) {
        if (!frm.doc.customer) {
            frappe.throw(__('Customer required'));  // Stops script, NOT save
        }
    }
});

// ✅ CORRECT — throw in validate prevents save
frappe.ui.form.on('Sales Order', {
    customer(frm) {
        if (!frm.doc.customer) {
            frappe.msgprint({ message: __('Customer required'), indicator: 'orange' });
        }
    },
    validate(frm) {
        if (!frm.doc.customer) {
            frappe.throw(__('Customer is required'));  // Prevents save
        }
    }
});

6. Recursion Guard with Flags

// ❌ WRONG — discount change triggers amount recalc, which triggers discount...
frappe.ui.form.on('Sales Order', {
    discount_percent(frm) {
        frm.set_value('grand_total', calculate(frm));  // Fires on_change loop
    }
});

// ✅ CORRECT — Use flags to break the cycle
frappe.ui.form.on('Sales Order', {
    discount_percent(frm) {
        if (frm.flags.skip_recalc) return;
        frm.flags.skip_recalc = true;
        frm.set_value('grand_total', calculate(frm));
        frm.flags.skip_recalc = false;
    }
});

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Debug Tools

Tool	How to Use	When
Browser Console (F12)	console.log(frm.doc)	Inspect form state
console.table()	console.table(frm.doc.items)	View child table rows
JSON.parse(JSON.stringify(frm.doc))	Deep-clone for snapshot	Avoid circular refs in console
frappe.boot.developer_mode	Check if dev mode on	Conditional debug logging
frappe.ui.toolbar.clear_cache()	Clear client cache	After deploying script changes
Network tab (F12)	Filter XHR requests	Inspect frappe.call payloads
frappe.show_alert({message: 'debug', indicator: 'blue'}, 5)	Visual debug in UI	Quick feedback without console

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ALWAYS / NEVER Rules

ALWAYS

1. Use the frm parameter — NEVER use cur_frm [v14+]
2. Wrap async frappe.call in try/catch — Unhandled rejections fail silently
3. Use __() for all user-facing strings — Required for translation
4. Collect multiple validation errors before calling frappe.throw()
5. Use frappe.get_doc(cdt, cdn) to access child table rows in events
6. Put frappe.throw() only in validate to prevent save
7. Check r.message for null before accessing server response properties
8. Use frappe.model.set_value(cdt, cdn, field, value) in child table events

NEVER

1. NEVER use alert(), confirm(), or prompt() — Use frappe.msgprint / frappe.confirm
2. NEVER expose stack traces to users — Log to console, show friendly message
3. NEVER use cur_frm — It is unreliable and undefined in many contexts
4. NEVER leave console.log in production — Use conditional frappe.boot.developer_mode check
5. NEVER mix .then() and await in the same function — Pick one pattern
6. NEVER call frm.set_value in setup — Form is not ready; use refresh or onload
7. NEVER ignore the error callback on frappe.call when using callback style

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Reference Files

File	Contents
references/examples.md	Real error scenarios with diagnosis
references/anti-patterns.md	Common mistakes with before/after fixes
references/patterns.md	Defensive error handling patterns